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String Trimmer (Strimmer) Won't Work: 2-Stroke Engine and Carburetor Troubleshooting

Updated on October 17, 2017
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Eugene is a qualified control/instrumentation engineer Bsc (Eng) and has worked as a developer of electronics & software for SCADA systems.

Typical gas string trimmer
Typical gas string trimmer | Source

String Trimmer Won't Start?

String trimmers, also known as line trimmers, weed eaters, weed whackers, strimmers, and whipper snippers, can be troublesome garden tools. Difficulty in starting is often due to an issue with the carburetor (carburator).

In this article, I offer a list of troubleshooting questions to help you find the problem. I strip and clean a carburetor and point out where problems can arise.

While I specifically deal with the carburetor from a trimmer, these tips will also help for carburetors on other two-stroke engine-powered machines (such as brush cutters, hedge cutters/trimmers, chainsaws, consaws, and leaf blowers).

Common Reasons Why a String Trimmer Won't Start

  • A dirty or cracked spark plug
  • The magneto isn't generating a spark
  • Improper venting of the cap on the gas tank
  • A blocked fuel filter
  • A very dirty air filter
  • Clogging of the carburetor from gum deposits due to gas being left in the tank over winter
  • A worn diaphragm or pump in the carburetor
  • A stuck valve in the primer bulb or a leaking bulb
  • A blocked spark arrestor screen

Usually though, the problem is caused by parts which wear out or become compromised in some way in the carburetor. Before jumping to conclusions and starting to troubleshoot, follow a proper start sequence.

Two-stroke carburetor.
Two-stroke carburetor. | Source

What's a Carburetor and How Does It Work?

All two-stroke engines on trimmers (and also four-stroke engines on lawn mowers, generators, and other small equipment) are fitted with a device called a carburetor (often shortened to carb in North America or carby in Australia). Engines in older cars and other gas-powered vehicles also had a carburetor before the introduction of fuel injection in the 1980s.

The function of the carburetor is to mix a mist of gasoline and air together to form an explosive mixture which is burned in the cylinder of the engine.

  1. Air is drawn by suction from the engine through a tube in the carburetor. This tube, about 1/2 to 3/4 inch in diameter on a small engine, has a narrowed section called the venturi.
  2. As air flows through the venturi, it increases in speed, and pressure drops below atmospheric. The drop in pressure causes fuel to be drawn through a jet into the air flowing through the venturi.
  3. This jet works exactly like the spray nozzle on a garden hose and a mist or spray of gas is created, which mixes with the airflow. (Paint spray guns used off a compressor work on the same principle). The mixture flows out of the carburetor and into the crankcase through the intake manifold.
  4. Finally, it is transferred to the cylinder where it is burnt to produce power.

Two-Stroke (Two-Cycle or 2T) Engines

Two-stroke (also known as two-cycle or 2t) engines are used on most hand-held garden power tools. They have several advantages over four-stroke engines (like the one in your car or lawn mower).

Firstly, there is no sump or reservoir on the underside, filled with oil for lubricating the engine. A sump would be impractical considering that such hand-held power tools may be used upside down and at all angles. Small four-stroke engines are fairly "primitive" and don't have an oil pump, relying on a "splasher" or "flicker" on the connecting rod to throw oil from the sump onto the piston and other moving parts. So this system would be ineffective on a two-stroke engine if it was used upside down.

To lubricate these type of engines, you need to pre-mix oil with gas. The absence of a sump and its oil content reduces weight. Also two-stroke engines don't have valves, a cam shaft, or the other bits and pieces which operate the valves. Instead, they have "ports" or entry and exit slots for fuel/air mixture and exhaust gases. This further reduces weight. So the result is a lighter machine, a higher power-to-weight ratio than a four-stroke, no requirement for an oil change, and less working parts to wear out or fail.

The greatest disadvantage of these types of engines is that because oil is burned in the combustion chamber or cylinder along with the gas, they produce smoke and are smelly (which causes pollution in cities where scooters and motorbikes are the main means of transport). Low smoke two-stroke oils are available, however, which somewhat cut down on emissions.

Two-stroke trimmer engine.
Two-stroke trimmer engine. | Source

How to Fix a String Trimmer

I have a bargain basement trimmer I bought in a large home store. Unfortunately, spare parts are unlikely to be widely available for repairing such a model (although the store may be able to get them from their supplier), which was probably mass produced in China. Still, it was cheap and I've got years of use out of it with no engine problems (except the protection guard for the line broke and I had to make a new one).

If you want a machine to last for 10 years or more, go for a model by a well-known manufacturer such as McCulloch, Stihl, Husqvarna, Poulan Pro, Oleo-Mac, Ryobi, Craftsman, Weedeater, Flymo, or Honda. That way, you can be certain of getting service information, spare parts, and after-sale support.

Warning: Gas Is Flammable

Gas (petrol) is flammable. Keep all sources of ignition away from where you are working on an engine. This includes naked flames, cigarettes, hot surfaces, and tools which generate sparks (e.g. angle grinders). Rags soaked in gas can ignite, so dispose of them safely. Store gas in metal cans or plastic containers made for the purpose.

How to Mix Fuel for Two-Stroke Engines

Don't use four-stroke motor oil in your engine. It contains additives which can contaminate the cylinder over time and also it can tar up the piston. Two-stroke oil is designed to burn as cleanly as possible with the fuel.

  • Lubrication of two-stroke engines is effected by mixing two-stroke oil with gas before use.
  • In general, the oil-to-gas ratio for a two-stroke engine should be 1:50. This equates to 20 ml per liter (litre) or 2.6 fluid ounces (US) per gallon (US).
  • Some manufacturers recommend a 1:25 ratio for their engines, so check your manual.
  • 1:40 or 1: 30 is probably a good compromise (i.e. more oil).
  • Try to avoid fuels which contain ethanol as this absorbs moisture over time, oxidizing metal surfaces.
  • Swirl the mixture in the container for a few seconds to mix it thoroughly.
  • You can normally buy 100 mL plastic bottles of two-stroke oil which is sufficient to make up 5 litres of mix. In the US, 2.6 fluid ounce bottles of oil are available for making up a gallon of mix. If you do a lot of trimming, obviously it's more economical to buy a larger bottle of oil.

Gas and Two-Stroke Oil Needed For a 50:1 Ratio

Amount of Fuel Mixture Required
Gasoline/Petrol
Two Stroke Oil
1 Litre
1 Litre
20 ml
5 Litres
5 Litres
100 ml
1 US Quart
1 US Quart
0.64 US Fluid Ounces (about 1.5 tablespoons))
1 US Gallon
1 US Gallon
2.6 US Fluid Ounces
Oil mix ratios. For a 25:1 ratio, double the amount of oil

Conversion of Capacity Units

1 US pint = 16 US fluid ounces

1 US gallon = 3.79 litres

1 UK imperial gallon = 4.54 litres

1 UK gallon = 1.2 US gallons

1 UK pint = 20 UK fluid ounces

Starting a Trimmer, Hedge Cutter, or Chain Saw Properly

Priming the Carburetor

  • Before you start your engine, you need to prime it. The priming bulb sucks fuel up into the carburetor which may be empty from the last time the engine was run.
  • The priming bulb must be pushed about ten times to suck in sufficient fuel. Make sure there is enough fuel in the tank before you do this so that the filter on the end of the fuel line is submerged and you're not just sucking air into the priming bulb.
  • The engine only needs to be primed again after a tank refill, if it has been a long time since the trimmer has been used and the carburetor has drained, or if the trimmer was used upside down for an extended period and the engine cut out.
  • The filter on the fuel line is normally able to move around in the tank so that even when the trimmer is used upside down, the filter falls down into fuel.
  • However, if there isn't much fuel in the tank, sometimes this doesn't work, so the intake line can get drained and filled with air as the engine uses up fuel, requiring a re-prime. Try to keep the tank at least half full.

Starting the Engine

  1. Rest the machine on the ground, clear of any debris, and start the engine on full throttle with the choke on.
  2. If it doesn't start after a few pulls, try re-priming.
  3. Once the engine gives a kick, give it another pull and if it runs, wait until it cuts out or for about ten seconds, whichever comes first.
  4. Then turn off the choke and allow the engine to continue running (or try starting again if it had cut out on-choke).
  5. If the engine won't start, repeat the process from the start by turning the choke back on, priming the engine, and pull-starting.
  6. If after several attempts at starting, nothing happens, the engine may have flooded. Leave it for about 10 minutes for fuel to evaporate before attempting a re-start.
  7. If the engine has been run in the last 10 minutes or so and is hot, you don't need to turn the choke back on before re-starting.

Note: There is some variability in the starting procedure for this type of engine. Some engines don't have a primer bulb. Also, an engine may have both a partial and full choke position. The choke may be arranged so that you start the engine without squeezing the throttle and revving it, only doing so when it fires up. Revving then automatically knocks off the choke. If possible, check your manual for details.

Try Pouring Some Gas into the Cylinder

  1. A traditional way of getting an engine to start is to pour a little gas into the spark plug hole. I emphasize a little, about a thimble full or a soda bottle capful. Try not to spill it on the outside of the engine.
  2. Screw the plug back in and start the engine on full choke. This often helps to get a stubborn engine running. Don't try this if the engine has been running and hot, because the gas could potentially ignite if you spill it.
  3. A less messy option, if you have a section of fuel or narrow gauge air line, is to dip it in gas to a depth of about 2 or 3 inches, put your thumb over the top, stick the end of the line into the plug hole, and take away your thumb, allowing the gas to run out.

Empty Fuel Lines Can Cause Difficult Starting

If you run the engine until it cuts out before re-filling, the fuel lines will have thoroughly emptied out. Remember you need to push the primer button 5 to 10 times before starting the engine again. If the check valve in the primer button isn't the best, this can cause difficult starting. If the button is working properly, it should feel "spongy".

Keep pressing it until it fills with gas and most of the air bubbles are gone. You can check whether the primer is working properly by partially filling the tank with gas but allowing the end of the line without the filter to remain above the surface (the line with the filter should be submerged). When you repeatedly push the primer button, gas should flow out of the line without the filter.

When undoing the spark plug, hold the wrench with two hands so that it doesn't slip off the plug and damage it.
When undoing the spark plug, hold the wrench with two hands so that it doesn't slip off the plug and damage it. | Source

Maybe There's a Problem With the Spark Plug?

Could be!

  1. Remove the plug with a spark wrench.
  2. Push the plug lead back onto it and hold the metal part of the plug against the engine body. This can be somewhat difficult to do with these engines, as a plastic cowl usually covers the engine and it can be difficult to access the metal body. You could use a bolt, small tin, or whatever to make contact.
  3. Switch on the starter switch, hold the plug by the rubber booth at the end of the spark lead (or maybe a clothes peg) and pull the starter cord/rope. This can be difficult to do single-handed unless you are an octopus, so find a trusty assistant to help you!
  4. A healthy spark should be blue. It might be difficult to see the spark in bright sunlight, so move the trimmer into the shade or indoors. If there's no sign of a spark, try a new one or even a plug from a lawn mower (to test the ignition circuit is okay) before you attempt any more in depth troubleshooting. (Don't use a plug which is longer than the original one as it could hit the piston!)

Use a socket or whatever to make contact with the body of the engine when testing for a spark.
Use a socket or whatever to make contact with the body of the engine when testing for a spark. | Source

Can a Dirty or Cracked Spark Plug Prevent Starting?

If the plug won't spark, there are two possible causes:

Cracked Plug

A crack in the ceramic insulation of the plug can cause the spark to take a "short cut" through the insulation so that it doesn't jump the gap where it's supposed to do

Dirty Plug

If the ceramic insulation which surrounds the central electrode is excessively covered in soot, again this can short out the spark so that it won't jump to the outer electrode (the one connected to the threads). You can clean soot off with a tooth brush, or better still a "toothbrush" style, small wire brush. Then scrub the electrodes using the toothbrush in a small bowl/coffee jar lid full of gas.

A "toothbrush" style wire brush is useful for cleaning soot from a spark plug.
A "toothbrush" style wire brush is useful for cleaning soot from a spark plug. | Source
Hairline cracks in the insulation of plugs can short out high voltage and result in no spark across the gap.
Hairline cracks in the insulation of plugs can short out high voltage and result in no spark across the gap. | Source

Could There Be a Problem With the Ignition Module?

As explained in great detail in my article Lawn Mower Won't Start? - Top 10 Mower Troubleshooting Tips, small engines have a device called a magneto for creating a spark at the plug to ignite the fuel/air mixture in the cylinder. This is a cross between a transformer and generator and creates voltages of about 10kV. These are normally electronic capacitor discharge ignition modules (CDI) on modern trimmers, so there are no points to be cleaned or a condenser (capacitor) to replace.

If you don't get a spark at the plug and have tried a new plug and checked for loose wiring without success, this module may be at fault and need to be replaced.

Alternatively, the embedded magnets on the flywheel may have weakened, or the gap between poles of the coils and flywheel may be off. This should be approximately 0.006 inches.

Electronic ignition module - When the kill switch is open circuit (on position), engine runs. Closing the switch cuts the engine.
Electronic ignition module - When the kill switch is open circuit (on position), engine runs. Closing the switch cuts the engine. | Source

Testing the Kill Switch

The kill switch shorts out the magneto to cut out the engine. Dirty or corroded contacts usually result in an inability to stop the engine rather than a problem with starting.

You can check the resistance of the switch with a multimeter set to the ohms range. If you don't know how to use one, read How to Use a Digital Multimeter (DMM) to Measure Voltage, Current, and Resistance.

This should change from infinite resistance when the switch is in the on position (when the switch is disconnected from the engine) to less than an ohm in the off position. Normally, a switch should have a high resistance when off, but remember in this arrangement, "off" refers to the engine switched off and the switch contacts closed.

Ideally, you should detach one of the leads from the switch when testing the resistance in the on position because the resistance of the ignition module in parallel will affect readings. If the resistance is several tens of kilo-ohms, however, this is okay because it means the switch is opening and not shorting out the ignition module.

Remove the switch with a screwdriver
Remove the switch with a screwdriver | Source
Set the meter to the "ohms" range. Resistance should be less than 1 ohm with the switch in the off position. Ideally you should detach one of the wires from the switch when testing.
Set the meter to the "ohms" range. Resistance should be less than 1 ohm with the switch in the off position. Ideally you should detach one of the wires from the switch when testing. | Source

Multimeters from Amazon

Useful for checking voltage, current, continuity, and fuses. You can buy a cheap meter like Innova which will do the job adequately, or alternatively a more expensive model like the Fluke 113 which is recommended for general purpose home/auto maintenance. By paying more you get better accuracy, plus the option of getting the meter calibrated/repaired.

What About the Cap and Check Valve on the Gas Tank?

Unlike lawn mowers and other such yard equipment which is normally used in one orientation, trimmers, chainsaws, and hedge cutters must be used at all sorts of angles.

A lawn mower has a vent in the gas tank cap to allow air into the tank as gas leaves it. If the tank wasn't vented to atmospheric pressure, this would restrict the flow of gas out of the tank. (Turn a large soda bottle filled with water upside down and you'll see what I mean).

55 gallon oil drums also have a secondary bung which needs to be loosened to vent the drum, enabling smooth flow from the main outlet during emptying. On a lawn mower, the vent is just a hole with a fiber or plastic baffle inside the cap to stop gas splashing out. This isn't good enough on a trimmer if the tank is turned upside down, so a one way valve (check valve) is used.

This valve may be fitted in the cap of the gas tank or on the body of the tank itself.

The valve allows air in but gas can't leak back out. This could get clogged with dirt, so try starting the engine with the cap slightly loosened (keep the tank upright) to see if it makes any difference.

To clean the valve, gently slide a small flat blade jeweler's screwdriver into it to make sure its clear (make sure it's small diameter to prevent damage). I couldn't get access to the back of this valve because a fuel filter was pushed into place. There wasn't a problem with the valve so I didn't go any further.

On my trimmer, the one-way valve is fitted in the gas tank cap.
On my trimmer, the one-way valve is fitted in the gas tank cap. | Source
This valve holder has a filter at the back. It seemed to be difficult to prise out and may have been bonded to the holder, so I left it in place.
This valve holder has a filter at the back. It seemed to be difficult to prise out and may have been bonded to the holder, so I left it in place. | Source

Could the Fuel Filter Be Dirty?

Possibly, although I've never had any problems with clogged filters on my hedge cutter, chain saw, or trimmer, and they have have had hundreds of hours of use.

Whether the filter gets clogged depends on the quality of the gas you get. If you don't take care when filling the tank and allow crap such as dirt and sand into it, it will inevitably end up in the filter. Impurities in two-stroke oil probably accumulate in the filter also.

If you can prime the engine and fuel can be seen to return to the tank via the line without the filter, the filter is unlikely to be clogged.

The filter is located on the end of the intake fuel line and is replaced rather than cleaned (although you could try removing it, wash in hot soapy water, rinse, and allow it to dry before replacing).

Magnetic parts tray. A cloth also stops parts rolling about.
Magnetic parts tray. A cloth also stops parts rolling about. | Source

Now It's Time to Remove the Carburetor From the Engine

Your carburetor may be somewhat different from the one pictured, but the basic principle is the same.

  • A section of membrane in the carburetor acts as a pump. Pulses of low and high pressure from the engine crankcase move this membrane backwards and forwards, and this sucks gas from the tank to a reservoir pocket in the carburetor.
  • Reed valves act as check/one way valves to stop gas flowing backwards.
  • A needle/metering valve operated by a diaphragm regulates flow into the reservoir pocket and shuts off flow when the pocket is full.
  • A primer bulb is included which helps to suck fuel up into the carburetor before starting. Without this, the starter cord would have to be pulled multiple times to fill the carburetor with gas. Some carburetors don't have this primer button arrangement.
  • It's a good idea to lay out parts on a piece of cloth or old towel to stop them rolling about during dismantling. You can also store them in a magnetic parts tray.
  • Don't work outdoors like I did while disassembling. This was only to facilitate taking photos in good lighting conditions. If you drop small parts on a lawn, you will probably never find them!
  • If you have an air compressor and blowgun, blow away any grime form the outside of the carburetor before removing. Do this before removing the air filter and fuel lines to avoid blowing grime into the carburetor or bursting the diaphragm inside it.

Drain the gas tank.
Drain the gas tank. | Source

First Remove the Air Filter

Remove the air filter from the housing and check it for dirt. Wash it in soap and hot water, then squeeze and allow to air-dry.

If you're just removing the air filter to clean it without doing any further maintenance or troubleshooting, it's always a good idea to close the choke beforehand to prevent any dirt from getting into the carburetor.

Filter Housing
Filter Housing | Source
Foam air filter.
Foam air filter. | Source
Ensure the filter is free of dirt. Wash if necessary.
Ensure the filter is free of dirt. Wash if necessary. | Source
The choke is simply a plate which covers the air intake. When it is closed, the engine sucks in a richer mixture (i.e a higher concentration of fuel in the air/fuel mixture). This makes for easier starting.
The choke is simply a plate which covers the air intake. When it is closed, the engine sucks in a richer mixture (i.e a higher concentration of fuel in the air/fuel mixture). This makes for easier starting. | Source

Remove the Air Filter Housing

Two nuts hold this housing in place.

Remove the retaining nuts with a socket wrench.
Remove the retaining nuts with a socket wrench. | Source
The carburetor exposed: Next, the fuel lines and throttle cable must be removed.
The carburetor exposed: Next, the fuel lines and throttle cable must be removed. | Source
Mark the fuel lines before removal. A "Tippex" style correction pen is useful for doing this.
Mark the fuel lines before removal. A "Tippex" style correction pen is useful for doing this. | Source
Don't stretch the fuel lines by pulling them. Instead, try pulling them gently while pushing with a flat bladed screwdriver or needle nose pliers against the edge of the lines.
Don't stretch the fuel lines by pulling them. Instead, try pulling them gently while pushing with a flat bladed screwdriver or needle nose pliers against the edge of the lines. | Source
Remove the end of the throttle cable.
Remove the end of the throttle cable. | Source
Carburetor ready for dismantling.
Carburetor ready for dismantling. | Source

CAUTION !!!

Never attempt to use a compressor blowgun to blow into any orifices/tubes before disassembling the carburetor. You may burst the diaphragm and gaskets!

Be Careful With a Blowgun

You can try cleaning surfaces/orifices with a blowgun, but never blow into any sealed compartments before disassembly to avoid rupturing the diaphragm/pump.

It's probably okay to blow over the outer surface of the assembled carburetor to remove grime, but keep the nozzle of the blowgun away from any openings.

Also remove any screen filters and loose parts first which could get blown away.

Wear safety glasses to protect your eyes from grime and small parts which could get blown up towards your face.

Carburetor Disassembly

The diaphragm and reed valve pieces are delicate so take care while handling.

It's also a good idea to take lots of photos at every stage of disassembly just like I did here, in case you forget how to put everything back together!

4 screws hold the carburetor together. Remove the primer bulb retaining plate.
4 screws hold the carburetor together. Remove the primer bulb retaining plate. | Source
Trimmers either have a primer button mounted on the carburetor or a remote primer.
Trimmers either have a primer button mounted on the carburetor or a remote primer.

Check the Primer Bulb for Holes

  • The primer on a two-stroke engine is used for sucking fuel up into the carburetor. Without the primer bulb, the starter cord would have to be pulled loads of times before the fuel lines and carburetor fill up.
  • Holes or splits can eventually form in the primer bulb. If this happens when the bulb is full, you'll probably notice it because gas will leak out. However, if it has been some time since the trimmer was used and the bulb has emptied out, any holes will likely prevent it sucking fuel up from the tank.
  • If the material of bulb has deteriorated, it's possible also that it won't seal properly against the body of the carburetor, resulting in an air leak. Again, this will prevent the bulb sucking up fuel.
  • A check or non return valve is part of the priming system (the red piece shown in the photos). This has two one-way sections. When you push the primer bulb, fuel flows from the bulb through the central one-way section to the carburetor (which, according to Wikipedia, is a duck bill valve). Fuel can't flow back because pressure squeezes the duck bill closed. Dirt in this valve can keep it open, allowing fuel to flow back to the primer bulb, so the carburetor doesn't get properly primed.
  • The outer section is known as an umbrella valve and the floppy outer edges prevent fuel flowing backwards. When you release the bulb, suction causes the flexible edges of the valve to rise and fuel enters the bulb. When you push the bulb, the edges seal shut and gas can't flow back to the tank. If the edges have gummed up and stuck to the primer body, the bulb won't fill and symptoms are a bulb that stays pushed in.

Primer bulb and one way "umbrella" check valve (red piece in center). Check the edges of the valve aren't stuck to the primer body.
Primer bulb and one way "umbrella" check valve (red piece in center). Check the edges of the valve aren't stuck to the primer body. | Source
One-way or check valve. Check there's no dirt stuck in the "duck bill" valve section in the center.
One-way or check valve. Check there's no dirt stuck in the "duck bill" valve section in the center. | Source

Fuel Flow Through Carburetor

With all the holes and passageways in the carburetor, this can be somewhat confusing to work out. Have a look at the photo below. This is the sequence:

  1. Fuel leaves the tank via the fuel line (the one with the filter) and enters the carburetor, flowing up through the screen filter at point 1.
  2. Fuel flows down passageway at point 2.
  3. Fuel flows back up at point 3. The reed valve prevents fuel flowing back.
  4. Fuel flows down at point 4 and into the compartment 5. Here it is pushed by the "pump" (which is just a flexible section of the gasket) to point 6. Another reed valve prevents it flowing back when the pump is sucking fuel from the tank.
  5. Fuel fuel flows into the reservoir pocket (on the underside of this half of the carburetor) via the valve at point 7. The needle valve in this brass piece is operated by the diaphragm (see explanation of diaphragm below).
  6. Finally fuel leaves the reservoir pocket via the cross shaped piece in the center, and sprays into the carburetor venturi through the jet (in the other half of the carburetor).

Reed valves and fuel pump. The red numbers are the sequence of ports through which the fuel passes, flowing up through the screen at 1, and exiting down through the metering needle valve at 7 to the reservoir pocket.
Reed valves and fuel pump. The red numbers are the sequence of ports through which the fuel passes, flowing up through the screen at 1, and exiting down through the metering needle valve at 7 to the reservoir pocket. | Source

Fuel Pump

Fuel is pumped by suction from the crankcase acting on a membrane. The pulses of pressure move the membrane backwards and forwards, sucking fuel. Reed valves prevent fuel flowing backwards. Handle this piece with care as it can be easily damaged.

Note: In the photo, this appears to be one piece. However if you zoom in, you can see that there are in fact two parts stuck together. Firstly a gasket (which makes contact with the numbered part of the carb) and secondly a part which has the valves and pump. When reassembling, it's important to replace these in the correct order to prevent leaks.

Carburetor Kits

If you have a trimmer made by a well-known manufacturer, the chances are that you will be able to buy a carburetor rebuild kit. This is a collection of parts which can be used to replace components in a carburetor which have become torn, worn out, warped, punctured, coated with hard deposits, or deteriorated in any other way to the extent that cleaning won't help. It usually comprises of gaskets, reed valve/pump piece, diaphragm, needle valve, springs, primer bulb, etc.

The other half of the carburetor. Part of the jet assembly is in the center.
The other half of the carburetor. Part of the jet assembly is in the center. | Source

Diaphragm on a Two-Stroke Engine Carburetor

Lawn mowers and other similar engines often have a float bowl and float. This acts in a similar way to a toilet cistern and ball cock valve, ensuring there is always a full reservoir of fuel at a constant level from which the jet is supplied with fuel. This is important for the speed behavior of the engine.

Unfortunately, a float bowl and float rely on gravity for correct operation and this system can't be used on a trimmer which is used in all orientations.

  • Instead, a reservoir compartment is built into the carburetor. As this empties, external atmospheric pressure acting on the diaphragm pushes the centre of it inwards. This forces a metering lever/rocker arm to pivot, lifting a needle valve up out of its seat, allowing fuel to flow into the reservoir pocket. Once the pocket fills, the process is reversed and the valve closes. So the pocket is always kept filled with fuel.
  • Just like the reed valve piece, the diaphragm is delicate and needs to be separated carefully from the body of the carburetor, especially if it has become stuck.
  • Over time, the diaphragm can become punctured or get stretched and "baggy" in which case the needle valve can stay shut, even when the reservoir pocket is empty. The material can also stiffen, preventing proper operation.

The diaphragm. As fuel is used, atmospheric pressure pushes down on the diaphragm, pivoting a rocker arm, lifting a needle valve and allowing more fuel to enter the pocket.
The diaphragm. As fuel is used, atmospheric pressure pushes down on the diaphragm, pivoting a rocker arm, lifting a needle valve and allowing more fuel to enter the pocket. | Source
Check for holes by holding up against a bright light source. Warping of these delicate parts can also prevent them working properly.
Check for holes by holding up against a bright light source. Warping of these delicate parts can also prevent them working properly. | Source
The diaphragm assembly consists of a needle valve, metering lever/rocker arm, and spring.
The diaphragm assembly consists of a needle valve, metering lever/rocker arm, and spring. | Source
Remove the retaining screw.
Remove the retaining screw. | Source
The needle valve seat. Clean this with carburetor cleaner.
The needle valve seat. Clean this with carburetor cleaner. | Source
Needle, metering lever, and spring.
Needle, metering lever, and spring. | Source

Clean All Surfaces

Clean all surfaces with a carburetor cleaning aerosol. This removes any gum deposits which can clog pathways and jets.

If you have a compressed air blow gun, you can use it to aid drying. Set parts aside to dry once cleaned. Plastic parts, especially the reed valve piece and diaphragm, should be dried immediately after cleaning as the solvents in carburetor cleaner can be harsh and dissolve the plastic.

Avoid bending the reed valves with the force of the jet by spraying gently or resting the piece on a flat surface.

If you find your trimmer leaks fuel when not in use, the problem can be caused by a damaged tip on the metering needle or an accumulation of gum. This can be removed with a cotton bud soaked in isopropyl alcohol (IPA).

Clean the fuel pathways with carb cleaner. Make sure the outlets are pointed away from your eyes.
Clean the fuel pathways with carb cleaner. Make sure the outlets are pointed away from your eyes. | Source

Long Lasting Trimmer Line From Amazon

This 0.095 inch diameter (2.4mm), 3 pound, 685 foot spool of professional trimming line by Oregon should last you years and years. According to Amazon, it has a high strength inner core and an outer coating to prevent welding. In other words, adjacent turns on a wound spool are less likely to stick together as the line unwinds from the spool. Check your trimmer to see whether it is compatible with this diameter line.

Clean all surfaces of the carburetor with carb cleaner.
Clean all surfaces of the carburetor with carb cleaner. | Source
Clean the jet. An air blow gun is invaluable for clearing a complete blockage in the the jet if you don't have any carb cleaner.
Clean the jet. An air blow gun is invaluable for clearing a complete blockage in the the jet if you don't have any carb cleaner. | Source

Re-Assembling the Carburetor and How to Reconnect Fuel Lines

Once everything has dried, re-assemble the carburetor.

  1. Sandwich the gaskets and diaphragm between the various sections of the carburetor, replace all screws and only barely tighten.
  2. Next, tighten the screws fully in a staggered sequence. Usually there's 4 screws, so lightly tighten diagonally opposite screws first, then the other pair of diagonally opposite screws, then fully tighten. Don't over-tighten because a carburetor is soft aluminum and it's easy to strip threads.
  3. If you forgot to mark the fuel lines, you can check whether you mixed them up by filling the tank so that only the line with the filter is submerged or pulling the line without the filter up out of the gas in the tank. What should happen when you press the primer bulb is that gas is sucked through the line with the filter on the end, through the carburetor and flows back to the tank via the other line without the filter. If the lines are mixed up, air gets sucked up into the carburetor and you will see this bubbling out through the filter.

Fuel lines in tank, the intake line has a filter.
Fuel lines in tank, the intake line has a filter. | Source

Blocked Spark Arrestor Screen in the Muffler (Silencer)

The muffler exhaust is fitted with a wire spark arrestor screen (so you don't set fire to your garden or start a bush fire!). Over time, this collects smuts and can get clogged, reducing power output of the machine. Low smoke two-stroke oil supposedly slows build up of carbon. If you add too much two-stroke oil to your gas, however, the engine will smoke and deposits of soot will accumulate more rapidly.

I have checked this screen on my trimmer every couple of years and have never seen any deposits worth talking about. In any case, you can clean the carbon deposits with an old toothbrush, compressed air, or a small wire brush.

Remove the cowling.
Remove the cowling. | Source
Remove the cover over the exhaust outlet.
Remove the cover over the exhaust outlet. | Source
Spark arrestor screen.
Spark arrestor screen. | Source
Clean the screen with an old tooth brush, wire brush, or compressed air.
Clean the screen with an old tooth brush, wire brush, or compressed air. | Source

Adjusting the Idle Speed

If the engine keeps cutting out when idling, the idle speed may be too low.

  1. Allow the engine to run at top speed for about a minute, then release the trigger.
  2. Turn the idle speed screw clockwise about one eight of a turn and wait to see if the engine continues to run without cutting out.
  3. If it still cuts out, turn the screw another eight of a turn.
  4. Some trimmers and all hedge cutters have clutches which prevent the cutting head from operating when the engine is idling. If the blade starts moving or the trimmer line starts spinning, turn the screw slowly back counterclockwise until movement stops.

Adjusting the idle speed.
Adjusting the idle speed. | Source

What If a Trimmer Won't Stop

If the start/stop switch fails on your trimmer (or other engine), turn on the choke. This will flood the engine and stop it promptly.

Trimmer Cuts Out on Full Throttle

  • Check the vent in the cap/tank isn't blocked.
  • Ensure cracks in intake fuel line in tank aren't sucking in air, preventing fuel from getting to carburetor.
  • Make sure choke is off.
  • Are you using the correct mixture? Use 1:50 or 1:40 oil:gas. Too much oil can cause problems.
  • Clean air filter and exhaust screen.
  • Diaphragm/pump section in carburetor may have stiffened so it can't pump/meter fuel.

Did You Get Your Engine Started Using the Information in This Article?

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© 2014 Eugene Brennan

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    • eugbug profile image
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      Eugene Brennan 6 weeks ago from Ireland

      Thanks A,

      I don't know what browser you're using, but in Firexox you can print the central text and photos without the adds and related stuff on the left. Pagination probably isn't the best though and some modules straddle pages.

    • profile image

      A Kearney 6 weeks ago

      A PRINT button (for article only) would be helpful, & appreciated.

      Best article that I have come across - many thanks!

      Will attempt some of your suggestions, prior to throwing my damn trimmer out in the garbage.

    • eugbug profile image
      Author

      Eugene Brennan 7 weeks ago from Ireland

      The model is SKU 325821

    • eugbug profile image
      Author

      Eugene Brennan 7 weeks ago from Ireland

      Hi Seamus,

      I assume you've tried a new plug and there's still no spark? Check also that the spark lead hasn't pulled out a bit from the rubber booth. Sometimes this isn't a moulded piece, the inner core of the spark lead just pushes onto the sharp end of the spring in the booth (that pushes onto the plug).

      I've sent Woodies A DM to see whether they can provide any details. I remember doing this before, but they couldn't help, but I'll let you know if they get back to me. In the meantime, I suggest you search for "strimmer ignition module" or "trimmer ignition module" on eBay and see if you can find something similar. These modules are likely made by the thousand in China and then fitted to unbranded strimmers which then get badged. Check also for any id no. on the module and search for that online.

    • profile image

      Seamus 7 weeks ago

      Hi Eugene,

      I have the same Pro-Cut strimmer as shown in your article. I have no spark, even with the kill switch disconnected. I have narrowed it down to the ignition module. I cannot get a spare part as Woodies import them from China without parts support and exchange the whole unit when they get a faulty one. Do you know of a supplier for spares? I get the feeling they are very like the Ryobi range. Is there an equivalent part?

    • profile image

      Tom 2 months ago

      Excellent article on 2 stroke small engines.

    • eugbug profile image
      Author

      Eugene Brennan 2 months ago from Ireland

      Hi Bev,

      Some things you can try:

      Check the choke is off, clean the air filter, and if you can remove the holding screws (don't force them if they are burned on), remove and clean the exhaust screen. Next make sure your two stroke mix is correct. So use 20ml of oil/litre in the petrol/gas (2.6 US fluid ounces per US gallon). Check the throttle trigger and cable is turning the appropriate parts at the carburetor. If none of this works, you may have to dismantle the carburetor and clean it and the jet, or possibly replace the diaphragm/gasket using a kit of replacement parts.

      Hope this helps!

    • profile image

      Bev 2 months ago

      When the the engine is on and it's revin theres not enough power getting to the bit to cut the grass what could it be plz

    • eugbug profile image
      Author

      Eugene Brennan 2 months ago from Ireland

      Thanks Kenneth! Glad you found it helpful!

    • profile image

      kenneth 2 months ago

      This was one of the best written DIY articles I have ever read. Very helpful and clear explanations and pics. Thank you!

    • profile image

      Kelly 2 months ago

      Brilliant and very detailed. And yes a girl finding this useful.

    • eugbug profile image
      Author

      Eugene Brennan 2 months ago from Ireland

      Hi Pharmon,

      That sounds strange. Are there definitely no blockages in the muffler or exhaust port? It could just be a coincidence that the trimmer refused to start after replacing the muffler and the engine might have flooded. You could try removing the muffler again and confirming it's causing a problem, but there is always a fire hazard with it removed.

    • profile image

      Pharmon 2 months ago

      Hello Eugene - I'm trying to resurrect my father-in-law's old McColluch 3227 trimmer after a decade of disuse. I cleaned out the carb and hoses and added a new spark plug but it wouldn't start. After I removed the muffler it started right up and seems to run fine. When I reattach the muffler it again won't start. I don't see any major deposits in the muffler, the airways appear open and the spark screen is cleaned. Any ideas why this could be occuring? Thanks.

    • eugbug profile image
      Author

      Eugene Brennan 3 months ago from Ireland

      Alan and Montego, It sounds as though both of you have a fuel feed problem. I suggest if you feel confident about it, that you check the diaphragm and gaskets in the carburetor to make sure none of the reed valves are stuck and that the metering needle is ok and opening properly. If the trimmers have a high speed adjustment screw, this could be clogged also. Have a look at this video:

      https://www.youtube.com/watch?v=LTlO5WN4RVM

      Two points on the video Firstly the guy doesn't dry any parts after using the cleaner. I've seen it recommended elsewhere that parts are dried off before re-assembly using an air compressor blow gun or tissue because cleaner can be harsh and damage some plastic parts if it's left on it too long. Also he tightens the screw adjustments after assembly, intending to readjust them afterwards. I would suggest that before you disassemble, you tighten the screws gently until they reach the end of their travel while counting the turns (including fractions of turns). Then you'll know how to get them back to where they were after cleaning.

    • profile image

      Montego 3 months ago

      My trimmer is getting fuel to the plug, fire to the plug and the on off switch is working but it won't crank. New fuel filter, fuel lines all connected properly, new plug. I'm using premix fresh gas. Its only two years old and I use it about 10 minutes per week, still looks like new. Primer bulb also working correctly and returning gas to the tank no bubbles. Its a Tanaka trimmer. Any help?

    • profile image

      Alan 3 months ago

      My strimmer engine will idle but wont rev on the throttle - when I press the primer bulb it will reve enough to turn the strimmer lines at low speed in short bursts until you press it again. Is it a fuel feed / blockage issue or a compression issue ? Any advice appreciated Thanks

    • profile image

      Aivars Loginovs 3 months ago

      Just reading the instructions you offer tells me I'm in the right place. I have just enough knowledge to be dangerous and not enough to troubleshoot logically w/o missing important steps. Thank you for helping the little guy. Must not make the big engine repair outfits happy. Now they can't so readily pick our pockets. Thank You

    • profile image

      gwk 3 months ago

      Done all that. I'm a motor tech and have gone through all the usual. Full gaskets New fuel lines and return. New primer and ducksbill valve. On priming there is ever only a half full primer bubble. On priming Air is being pumped back to tank. No Air in uptake line. Air is getting into circuit somewhere. I'm flumoxed . Any ideas? Thanks for your prompt reply.

    • eugbug profile image
      Author

      Eugene Brennan 3 months ago from Ireland

      When you prime, fuel should return to tank. Sounds as though air is somehow getting into primer bulb area and then being pushed back out return line. When you push bulb, it should feel spongy. Check also for any cracks in fuel lines which could be letting in air. Fuel filter should fall down properly into fuel. If it's tangling on something and not dropping, air will get sucked in.

      Check all your gaskets are seated properly and tighten screws in a staggered sequence.

    • profile image

      gwk 3 months ago

      Hi. working on April's strummed. Initially suffered from fuel starvation. Replaced tank vent. No go. Overhauled carb gasket set. No go. Replaced duckbill valve. No go. On priming fuel I have pure petrol in line but loads of air in return line. Lines are new and not reversed. Air is being drawn in, but where. Any idea?

    • eugbug profile image
      Author

      Eugene Brennan 4 months ago from Ireland

      Hi Mark,

      In answer to your questions:

      1) Until now I have only tried to clean the carburetor. The first time, I assumed that was the only option I had. Then I identified the carburetor and now know that gasket and rebuild kits are available for it. I was much more careful during the second cleaning and did a better job, particularly with the strainer. (I thought it might be my problem). I thought all parts looked reusable, no visible damage. The only area that wasn't cleaned directly is below the Welch plug. Could that be the achilles heel in my process?

      A1) The idea of Welch plugs is to cover up "junctions" between machined ports (the passageways can't be totally cast). As far as I know Welch plugs need to be replaced after removal, but you may get replacements with a carburetor kit. If you have an air compressor, you could try blowing through a port with an air gun and confirm air exits through the other port (don't blow into a sealed part such as the diaphragm reservoir/pocket without disassembly because it could cause damage). This would indicate the port is clear.

      2) I haven't replaced the fuel cap. If the carburetor now has no priming issues, shouldn't that verify that the fuel cap is working properly?

      A2)It should do because if air isn't being vented into the tank as fuel leaves, priming would probably fail. The primer bulb should fill with fuel if it's working ok, and fuel should return to the tank via the line with no filter.

      3) Does adding fuel at the spark plug hole effectively bypass the carburetor? If so, and the engine still doesn't start, what does that mean?

      A3) The idea of putting fuel into the cylinder is to bypass the carburetor and hopefully some of it will evaporate and explode, warm the cylinder and aid starting. However it can also flood the engine.

      4) What should my next step be? I am at a loss as to where to begin.

      A4)

      You can put some light oil on recoil starter springs, but don't oil rope/pulleys. If the starter pulley is nylon, it already has a low coefficient of friction (it's a slippy plastic) and doesn't need to be oiled.

      This is a guide for replacing the pull cord on a trimmer:

      https://hubpages.com/living/Stihl-KM-56-RC-Trimmer...

      Hopfully the plug is the correct length and isn't hitting the piston? In any case when you replace the plug, it will be more difficult to pull the starter cord because of the resistance as the mixture is compressed in the cylinder.

      If you've looked at the animation on the "Animated Engines" website (See link at the top of this page), you probably will have discovered how the crankcase is used to store mixture before it gets passed to the cylinder. On my trimmer 4 screws hold on the crankcase and it's fairly well sealed. I had a Homelite chainsaw and the gasket was pretty crappy, so that's something to checkout.

      You could try removing the spark lead, take off the air filter and put your palm over the intake manifold while pulling the starter cord. If fuel is getting through the jet, this should flood the carburetor. If there's no sign of fuel, there's something seriously wrong with the pumping mechanism or meter needle. Don't try starting this way because there's a potential fire risk if the engire backfires.

      Make sure you're starting the engine properly. So prime, pull start with choke on until engine runs or pops, then turn choke off after a few seconds and allow to run, or if not running pull with choke off. Two stroke engines are easy to flood and you may have to leave the engine for 15 mins or so until fuel evaporates. (removing plug make this happen quicker)

    • profile image

      Mark Harrison 4 months ago

      Hi Eugene,

      Struggling to get my Homelite Trim N' Edge String Trimmer, Model UT-20702, working again. Looks similar to your model with red housing. Here's my process so far:

      Phase 1: When I decided to take a look at it, the gas lines dissolved at the wall of the fuel tank. I replaced them and the fuel filter as it was integral to the replacement lines in the Ryobi kit I bought. I might have compromised something by removing the tank when replacing the lines, but was careful to reassemble cleanly. Found the Primer bulb was leaking from a split around the top and replaced it as well. Also cleaned the air filter. After reassembled, carburetor primed, but the engine would not start.

      Phase 2: Cleaned the carburetor as per your instructions. Didn't do a good job as now the primer would not pull gas from the fuel tank.

      Phase 3: Cleaned the carburetor a second time. Between the cleanings, was able to determine that the carburetor is a Zama C1U-H39A. After this cleaning, the starting problems continue.

      Phase 4: Confirmed that I had spark at the spark plug, also cleaned it and checked the gap, which was correct. Then replaced the plug with a new one and still no start. Tried adding gas to spark plug hole and still no start.

      Observation: The string pull is difficult and noisy. Noticed that the pull is smooth with the spark plug removed. Attempted this several times and got puffs of smoke through the spark plug hole. Once the plug was reinstalled, pulling the string becomes difficult again.

      Phase 5: Took a look at the spring and pull. Think I've lost some of the spring tension during this process and need to retighten. Was wondering about lubrication for this as there appeared to be none present. Can you tell me how to reinstall the spring at full tension? Not real obvious to me.

      Questions: 1) Until now I have only tried to clean the carburetor. The first time, I assumed that was the only option I had. Then I identified the carburetor and now know that gasket and rebuild kits are available for it. I was much more careful during the second cleaning and did a better job, particularly with the strainer. (I thought it might be my problem). I thought all parts looked reusable, no visible damage. The only area that wasn't cleaned directly is below the Welch plug. Could that be the achilles heel in my process?

      2) I haven't replaced the fuel cap. If the carburetor now has no priming issues, shouldn't that verify that the fuel cap is working properly?

      3) Does adding fuel at the spark plug hole effectively bypass the carburetor? If so, and the engine still doesn't start, what does that mean?

      4) What should my next step be? I am at a loss as to where to begin.

      Thank you very much for your informative articles.

    • eugbug profile image
      Author

      Eugene Brennan 4 months ago from Ireland

      Hi,

      If you imagine the valve in an upright position like a real umbrella, fuel flow should come up from underneath the perimeter of the "umbrella", flow across the top and then back down through the central "handle". From that you should be able to work out the direction of flow.

      Check when you prime that fuel is actually getting up into the carburetor and returning to the tank. You can do this by keeping the line without the filter on the end above the fuel level. If that works, the problem is likely to be with the diaphragm or fuel pump.

    • eugbug profile image
      Author

      Eugene Brennan 4 months ago from Ireland

      Question received via email:

      I have managed to clean and rebuild my unit but it fails to run for more than a few seconds when it does start.

      I notice that there are 2 flow paths in the housing that holds the "umbrella" / duck bill check valve that appear to have one way valves fitted.

      In what direction should the flow be in these paths as 1 of mine appears to be blocked.

      I can freely use an aerosol cleaner in all ports and paths except one in the "umbrella" housing.

      The electrics all appear fine, with a good spark at the plug.

      It also has good compression so must be fuel related trouble.

      Thanks for your help in advance.

    • eugbug profile image
      Author

      Eugene Brennan 5 months ago from Ireland

      Hi Schlankae,

      The problem is likely a fuel delivery issue due to the carb not being able suck fuel when the choke is open. Here's a few things you can try:

      Replace the fuel filter if you haven't done so already when you changed the lines.

      Check the vent in the tank cap/tank isn't blocked.

      Check the metering needle isn't stuck and if it has a rubber tip, this isn't damaged. Make sure passageways in the carb aren't blocked.

      Examine the diaphragm to see whether it has any holes and make sure it isn't deformed. Either scenario would require a replacement. Replacement carb kits are inexpensive and widely available from Amazon/eBay.

    • profile image

      Schlankae 5 months ago

      Hi, I have a Ryobi trimmer that I hadn't used in *several* years. I finally decided to get it running again, and I've replaced the fuel lines and primer bulb. I actually managed to start it, but only with Choke on. Initially it ran fast/slow which seemed to even out the longer it ran. However it only runs with choke on. As soon as it turn off Choke to "Run", it cuts out. It restarts immediately with choke on and I can play with the choke lever, but the closer I get to Run (choke off) it cuts out. Any suggestions?

      Thx!

    • eugbug profile image
      Author

      Eugene Brennan 5 months ago from Ireland

      Hi Davy!

      I doubt whether he ran the strimmer on pure diesel. A 4 stroke engine (e.g. a lawn mower engine) will run on kerosene when warmed up, but the fuel doesn't burn fully when the engine idles and the result is clouds of smoke from the exhaust due to unburned fuel. It probably clogs up the engine with soot over time also and can cause damage due to "knocking". Petrol-paraffin tractors used to run on TVO after WWII which was paraffin with waste additives added back in to increase the octane rating. Other petrol engine tractors ran on kerosene with heaters to vaporise the fuel before it got sucked into the engine. Diesel is less volatile than kerosene but a better lubricant, so your friend's logic was probably to use it in lieu of two stroke oil, but maybe he was overenthusiastic and added too much?

      Petrol should flush everything out eventually, but first I would drain any remaining fuel out of the tank. Then keep pushing the primer bulb while checking the return line in the tank (the one without the filter) until no more fuel comes out. Then add mix with a 1:50 ratio as described above. If that doesn't work, follow the starting procedure I describe in the article. So prime about 10 times, start on choke until the engine kicks, pull again and if the engine runs, allow it to run until it cuts out and then start again with no choke. There's a bit more detail on starting above, so check that out.

      Hope that helps, but go through the troubleshooting steps and see if everything else checks out ok.

    • profile image

      Davy Collins 5 months ago

      Hi Eugene

      I was recently given a Strimmer from a friend as he had problems starting it so bought a multi tool version instead. He'd had it served before and working so really should not had a problem!? The curious thing was he said it ran on Red Diesel which I never heard of before. On checking online its a DynamAc DB-26 2 Stroke engine so how can I purge the engine and carb etc of the Red Diesel if that whats in the engine? Would explain why doesn't start? Thanks

    • profile image

      scott 5 months ago

      nice job.Thanks

    • eugbug profile image
      Author

      Eugene Brennan 7 months ago from Ireland

      Hi bps001,

      It sounds like a fuel starvation and feed problem. If you press the primer bulb and it improves matters temporarily, this seems to indicate that the carburetor isn't getting enough fuel. An engine may idle ok when the throttle is almost closed and a small amount of fuel is spraying into the airflow. However when the throttle opens up and lots of air rushes in, if the jet isn't supplying sufficient fuel, the fuel/air mix will be much too lean (weak) and the engine will stall.

      Firstly eliminate any simple problems. So check the choke is off, clean the air filter, and if you can remove the holding screws (don't force them if they are burned on), remove and clean the exhaust screen. Next make sure your two stroke mix is correct. So use 20ml of oil/litre in the petrol/gas (2.6 US fluid ounces per US gallon). Check the throttle trigger and cable is turning the appropriate parts at the carburetor. If none of this works, you may have to dismantle the carburetor and clean it and the jet, or possibly replace the diaphragm/gasket using a kit of replacement parts.

      Hope this helps!

    • profile image

      bps001 7 months ago

      Hi Great article.

      I have A Titan strimmer and it not working probably. It seems like there is not the right amount of fuel going into the engine. I can get it start but when I go to used the strimmer or any of the attachments it dose not seem to have the right amount of power to make it move correctly as when you use the strimmer attachment you can see it is not spinning right. But when you have it running and you press the prime bulb it seems to have more power and then it works like it should but only for a couple of seconds. How do you could fix this?

      Thanks

    • eugbug profile image
      Author

      Eugene Brennan 7 months ago from Ireland

      Good! Glad the guide was of use to you Donna. Happy Trimming or (or Strimming as we say here!)

    • profile image

      Donna 7 months ago

      You are brilliant, Eugene! I followed your exact instructions and it started right up. I am so glad I found your website. I was prepared to clean the carburetor and now I don't have to, thanks to you!!! I really appreciate you sharing your knowledge and for taking the time to respond to specific questions. Thanks again.

    • eugbug profile image
      Author

      Eugene Brennan 7 months ago from Ireland

      Hi donna,

      It may have gummed up but what can happen sometimes is that fuel can slowly leak into the carburetor and then into the venturi or intake airway and flood everything. When fuel evaporates, lots of two stroke oil is left behind which would explain the smoke (usually blue or white).

      Try removing the air filter and if it is saturated with oil residue, wash it in soapy water and squeeze dry (only applicable to foam filters). Also try pull starting several times on no choke to clear any fuel which may have accumulated. Then start as normal using the procedure I have explained in the article. At the start of the season it can take a lot of choke on/choke off plus reprime attempts to get everything working again.

    • profile image

      Donna 7 months ago

      Thank you for such an informative article. I have an Echo PE-225 edger. I had not used it for about 8 months and when I tried to start it, it blew a lot of smoke. So, I drained the fuel and replaced it with fresh. Now it won't start at all. Do you think I gummed up the carburetor? If so, do you think I could clean it myself? I don't have a lot of experience with this sort of thing but am willing to give it a try. Thanks again.

    • profile image

      Denny M 7 months ago

      Great informative article!! Read every word and will use this information on my craftsman leaf blower. I baby this machine, but lately requires a shot of carb cleaner in air intake to get started and then runs perfectly. This with a new carb and filter. Anyway, great info. Thank you.

    • eugbug profile image
      Author

      Eugene Brennan 13 months ago from Ireland

      Thanks Mike. Possibly I have a gasket also on top of the diaphragm on my trimmer, so I must check this out. The kits have 6 parts so do you mean the diaphragm and pump section are both sandwiched between a top and bottom gasket?

    • profile image

      Mike 13 months ago

      I'm sorry I ment to say that in my case I needed to put another gasket on top of the diaphragm and the pump/reed with the aluminum section in the middle

      Thanks again and I really appreciate an author who actually reads and responds to the comments. Often being the best part of reading blog posts on line

    • eugbug profile image
      Author

      Eugene Brennan 13 months ago from Ireland

      Yes, carburetor kits seems to be readily available for the trimmer in the photos above (well at least they look similar, so you can only buy and try). Search for "K20-WYJ Walbro repair kit" and you should get plenty of hits on eBay and Amazon. The websites listed at the end of the article also supply this kit.

    • profile image

      Mike 13 months ago

      You're right about the pictures that's always the best way to reassemble. Thank you and after a lot of online research you are definitely right about the gaskets touching the centerpiece in which you number and then the diaphragm low read valve above

      Thanks again and for anyone looking to buy a new gasket set what you want to search for its walbro wyl series carb

      rebuild kit

      on Amazon for about 7 Bucks comes with everything from air filter gasket to primer bulb also includes the metering pin

    • eugbug profile image
      Author

      Eugene Brennan 13 months ago from Ireland

      I've added a note in the text about the two parts stuck together. If you zoom in on the photo (zooming is awkward on HubPages. You need to left click to zoom, then right click, click view image from the menu, then left click again) you can see that the gasket section makes contact with the numbered part of the carburetor. I don't remember mixing them up, and I'm always particular about remembering assembly sequences (don't have a good memory, but a camera is a good substitute!)

    • eugbug profile image
      Author

      Eugene Brennan 13 months ago from Ireland

      Hi Mike, thanks for the appreciation!

      If you look at the photo where I'm marking the fuel lines with the Tippex marker, it gives an idea of the assembly sequence (but you have to look at the other photos too to work everything out). I take your point that an assembly photo (something like an exploded parts diagram) would be useful. Anyway this is the sequence:

      1: The bulb goes at the bottom

      2: On top of that the primer bulb

      3: Next the black plastic part that the bulb fits into

      4: Followed by the diaphragm,

      5: Next the aluminum part holding the spring/meter valve

      6: The gasket

      7: The pump reed valve piece (6 and 7 are stuck together in the photo above)

      8: The carburetor section with the venturi which bolts onto the engine block

      I never noticed the part with the reed valves is two separate sections (stuck together in my case, and looking at photos, I can see the tabs from the two pieces). If the pieces are swapped, the tabs still wouldn't line up. However, the gasket seems to make contact with the part of the carburetor I numbered, followed by the pum/reed valve section.

      I don't know why the pieces are separate, presumably the gasket needs to be thick to fill imperfections in the machined surfaces, but the reed valves and pump have to be thin to sit flat on the ports and be flexible enough to flap up and down, plus the pump has to be thin to bulge in and out easily.

      It would probably be a good idea to give the photos numbers, so you've given me some work to do this afternoon!

    • profile image

      Mike 13 months ago

      Just to further clarify the above question in the picture of the pump/reed valve gasket you'll notice that there are 2 tabs at the bottom of this - each tab is a separate or in my case now a separate gasket. I have the same carb as shown, which has the 2 pin holes to help align everything, but when those 2 gaskets which are shown together in the picture are separated, one could be applied on the top of the other and still line up. Leaving me with the question of why didn't I pay attention to what one goes on the other.

      Thanks again, and side note - with your info and while waiting over the weekend for my new carb I was able to get my little 2 stroke scooter running enough to get me to the corner store and take the dog for a run..... with a zip lock bag primer bulb and a plastic tape over hole pump gasket

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      Mike 13 months ago

      I've spent several days trying to get information on this type of carb via Google and YouTube and nothing has come close to how informative your article is. I realize that it was written a couple years ago and I don't often make online comments but I wanted to thank you to show my appreciation for the hard work and time you put into writing this. This has been the only information I found on how the gas actually flows through the carburetor and I really appreciate the image showing the numbering sequence. that's exactly what I was looking for in order to figure out what my trimmers issue was. The only thing I still do not know and probably the only thing I could even remotely suggest adding 2 this article, : should the outer gasket go above or below the pump/reed valve gasket? meaning if the primer bulb is the bottom of the carb then the red-duck valve then the diaphragm then the part of the carb with the spring and meter.... To simplify - the picture in which you number the sequence of gas flow: Should the (outer) gasket go directly on top of that part of the carb and then the pump/Reed valve gasket... Or should it be the pump/Reed valve gasket and then (outer) gasket on top? In the picture with the gas flow numbers - on the left, the pump gasket and what I'm calling the outer gasket referring to the 2 tabs at the bottom. It would be greatly appreciated if you could reply or if anyone else knows the answer. I understand it's been a while since this was posted so thank you or any others in advance

      Mike

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      Eugene Brennan 13 months ago from Ireland

      Thanks Eduardo! I wasn't familiar with the name "whipper snipper" and to Google it and it turns out that's what trimmers/strimmers are called in Australia! Anyway thanks for the comments. If you're only doing light trimming, an electric machine is perfectly adequate and there are electric versions available with powers up to 1000 watt. That's well over 1 horse power, and they take a 2mm line.

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      Eduardo 13 months ago

      I have problems with the primer/fuel line(s) of a McCulloch whipper snipper and I was considering the replacement for an electric one, but after reading your very professional description of the fuel system in a two stroke engine and the cleaning of the carburetor, I will give it another go replacing the hoses, cleaning the filter and carby ; your article is -by far- the best I've read about a small two stroke engine in the web, thank you very much!

      Eduardo

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      Nick Mifsud 13 months ago

      Yep, first paragraph makes sense...the shaft had been rotating slightly relative to the the engine casing...

      So that's next weekend's activity sorted...

      Thanks, as ever, for the advice....

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      Eugene Brennan 13 months ago from Ireland

      Hi Nick, sounds really frustrating!

      So you've replaced the carb, plug and air filter, isn't really much left to trouble shoot!

      I wonder has the flywheel moved on the shaft which would screw up timing and cause the spark to happen at the wrong time? I've never removed a flywheel on a small engine like this or checked how they are fixed to the shaft, but presumably it's the same as on a lawn mower with a slotted key way on both components and a key to prevent relative movement. If the key has sheared, the flywheel may be held tightly enough by the nut to stay in the right position some of the time, but then move and cause problems. However for the engine to have started properly after you replaced the carb, it would have to have moved back to the correct angle. You would probably need a pullers to remove the flywheel though. The bolt/nut holding on a flywheel, mitre saw blade, circular saw blade etc always loosens in the direction that the blade/disk/wheel turns.

      The only other things I can think of are an ignition coil module which is breaking down or maybe the gasket on the crankcase giving trouble. Crankcase compression is necessary for proper operation. (See the link at the start about how a two cycle engine works). Also check the lead that feeds into the spark plug boot is making a good connection. These aren't actually crimped/spot welded/soldered to the spring which pushes onto the spark plug. Instead there's just a spike on the spring which pushes into the end of the HT cable.

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      Nick Mifsud 13 months ago

      Hi Eug - an update:

      After erratic behavior I ordered a new carb from eBay, popped it on and it started working almost immediately. After it stalled, I managed to start it again: success. (At this stage, new carb, plug and air filter).

      Next day: back to square one.looked like it was going to start with choke on, choke off and nothing. Managed to get it going once briefly but that's it.

      Since then stripped and reassembled, checked spark, fuel flow looks okay when priming.

      Baffled...

      Am I missing something really obvious?

      Nick

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      Eugene Brennan 14 months ago from Ireland

      Is the engine still hard to turn Nick? I'm working on an engine at the moment from a cement mixer (actually it came from a lawn mower, but that's a long story!). Anyway engine died during mixing concrete (very frustrating). Then I spent day and days trying to fix it. It would run ok for about 15 seconds and then cut out. After being left for a few seconds, it would run again but keep cutting out. When it cut out, it seemed to be difficult to turn and squeaked a bit. So I dismantled the engine and it turned out that the oil flicker, a sort of scoop device attached to the connecting rod and used on most small engines to throw oil over everything, had broken off. So the crankshft/connecting rod "bearing" wasn't getting oiled and it only took about 15 seconds for everything to heat up and seize from aluminium getting smeared over the moving surfaces. If your engine is cutting out, it could possibly be seizing if some of the moving parts are jamming up when hot.

      You could try getting a carb rebuild kit either on eBay (search for stihl + fs120 + carb + kit) or from Garden Hire & Spares Ltd in the UK (see link at end of article).

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      Nick Mifsud 14 months ago

      Now I'm really confused: back home this weekend and the machine started! It wasn't idling well and once it's gone out I can't restart.

      I've increased the idling speed a touch and will try again tomorrow, but suspect still have the ORIGINAL issue..i.e. Can't restart it after it goes out after running even for a short while (30 seconds).

      Conclusion: new carb required?

      Nick

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      Nick Mifsud 14 months ago

      The plug wasn't the issue...but I changed it because I had a spare...

      I've tightened everything up....

      ...suspect I'm going to admit defeat and take it in...:(

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      Eugene Brennan 14 months ago from Ireland

      Hi Nick, I think it's becoming an epic at this stage!

      Is the flywheel now turning ok with the plug replaced? Do you think the plug could be hitting the piston? I had another brainwave that maybe there was an issue with a decompression valve not working, but it seems the FS 120 doesn't have one of these valves fitted.

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      Nick Mifsud 14 months ago

      The saga continues.

      Flywheel turning okay, plug okay and sparking but no ignition, even though fuel seems to be coming through.

      Added a cap full of petrol into the cylinder (just to check) and replaced the plug...still no sign of life...

      Take it I've a compression issue...or am I missing something else...?

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      Eugene Brennan 14 months ago from Ireland

      Just had a thought, the plug could be hitting the piston and preventing it from reaching the top of the cylinder. Did you by any chance use a longer replacement plug? Also if you forgot to replace the washer on the plug, the extra couple of mm could be allowing it to reach further into the cylinder.

      Is the flywheel hard to turn just at one point in its circle, or right through 360 degrees?

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      Nick Mifsud 14 months ago

      Thanks for the quick reply! I'm in Scotland btw...

      I'll check I've got the correct gap tomorrow, I think I do since it turns just fine when the plug's out and the spark is good...

      Like you say, no sign of too much fuel...but I'll double check in daylight.

      Thanks again...

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      Eugene Brennan 14 months ago from Ireland

      Hi Nick,

      Is the ignition coil definitely tightened on its mountings and not rubbing against the flywheel? (the magnet could still be pulling it in as it passes). The combination of friction from a rubbing coil plus the force required for compression in the cylinder could be making the whole thing difficult to turn.

      Before you tighten the coil, you should slide something in between it and the flywheel magnet. The gap is supposed to be between 0.2 and 0.3 mm or the thickness of a business card. I don't know where you're located Nick, but if you're in Europe (assuming you are because you call it a strimmer!), Lidl often sell vernier callipers which are a very handy measurement tool for checking thickness/size of big and small things. Anyway press the coil gently against the spacer and tighten. Then replace the spacer. The only other thing which would make the flywheel hard to turn is a hydraulic lock when the cylinder partially fills with petrol. Since fluid can't be compressed, the piston wouldn't move. However you would notice all this petrol when removing the plug.

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      Nick Mifsud 14 months ago

      Hi Eug,

      Back after a 4 week spell away.

      So worked out the flywheel problem: I hadn't replaced the ignition coil box firmly enough and so the magnetic force had wedged it into the flywheel.

      But when I tried to turn it again it wasn't turning smoothly. Removed the spark plug and it turned fine and I have a good spark.

      Replaced spark plug and same problem - difficult to turn and no sign of it starting.

      Any ideas?

      Thanks,

      Nick

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      John keating 14 months ago

      Thank you Sir this article has been very helpful.

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      Nick Mifsud 15 months ago

      Thanks...looks like there never was an arrestor plate fitted!

      Return starter seems fine. It's the flywheel that simply won't budge (it moved a little before it stopped moving at all). Really weird as I didn't touch it at all...

      Suspect next step (next weekend) is to take it off and see what's going on behind it...

      Nick

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      Eugene Brennan 15 months ago from Ireland

      Hello Nick, page 9 of this service manual shows the spark arrestor:

      https://www.scribd.com/doc/140934817/FS120-FS200-F...

      Page 30 has some details on the recoil starter and they recommend lubricating with a few drops of kerosene. I would remove the complete starter from the engine and investigate whether it's the starter which has seized up and wont turn, or the engine. It's always a good idea also to wear safety glasses if you're doing any work on a starter, e.g. when replacing a rope. If the spring pops out suddenly, it could cause a serious eye injury.

    • Nick Mifsud profile image

      Nick Mifsud 15 months ago

      Hi Eugene,

      Back home. Stihl FS120 rubbish today so I stripped it down and cleaned it.

      First problem: can't find the spark arrestor screen anywhere (suspect this might be the culprit).

      But major issue: the starter rope won't turn the crank shaft, which has very little movement either way. Really confused.... Am I missing something obvious?

      Thanks,

      Nick

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      Eugene Brennan 15 months ago from Ireland

      My hedge trimmer has similar symptoms, when it's idling, speed slowly drops off and a nudge of the throttle or primer bulb keeps it going for another while. I still think it's something to do with the fuel pump or possibly the fuel filter is blocked (priming would charge the carburetor with fuel, but the pump might be unable to maintain the diaphragm "reservoir" in a filled state and it would eventually empty. However have you tried running it with the cap slightly loosened to eliminate a problem with the valve in the cap?

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      Nick Mifsud 15 months ago

      I blew air into the fuel cap to clear a possible blockage in the valve in the cap which might have been causing suction...

      Machine idles fine and I've kept the clutch well greased.

      One other thing...as I sense it's about to conk out (power drops off) a couple of nudges of the primer bulb keeps it going for another 5 to 10 mins...does that provide a clue?

      Priming the engine does not help after it's gone out and when it's still hot.

      Nick

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      Eugene Brennan 15 months ago from Ireland

      If blowing some fuel into the cap works, it sounds like a fuel pump problem, the air possibly helping to force fuel into the carburetor. Don't (I've learned the hard way!) blow compressed air into a carburetor before it's dismantled because it could burst the diaphragm or do other damage.

      Engines with clutches driving the cutting head can conk out when idling if the springs in the clutch are dodgy, causing the shoes to move out erratically, stalling the machine.

      Does priming the engine when hot make any difference and help starting?

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      Nick Mifsud 15 months ago

      Definitely no choke on and it starts fine when it's cooled down.

      It used to conk out quickly after it got warm and I thought blowing some air into the fuel cap seemed to help, it goes much longer now (about 25 mins)...so I guess it could be related.

      Now I've the confidence, I'll strip it down and give it a good clean.

      Nick

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      Eugene Brennan 15 months ago from Ireland

      Hi Nick, thanks for the comments!

      Is the choke definitely off when you try to re-start? I haven't come across this problem personally, but it sounds like the carburetor is flooding. Another suggestion I've come across is that pressure could be building in the tank. This wouldn't be so much an issue when the machine is running but once it stops, pressure would force fuel past the needle flooding the carburettor. You could try loosening the tank cap and see if it resolves the problem. Then checkout the vent in the cap or tank to see if anything is clogging it. It's possible also that the plug is sooting up from using fuel with excess two stroke oil in it. Again the engine might run ok, but once it stops, the spark could be too weak to get it going again. Apart from that, the issue could be with ageing of the carburetor pump/diaphragm. This might stretch/warp as it gets hot so that it doesn't work properly.

      Can you re-start the brushcutter when its allowed to cool down?

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      Nick 15 months ago

      Eugene, you are more hero. Cleaned the carb like you said, needed to replace one of the fuel lines, but my budget strimmer seems to be running again.. Thanks so much for your clear instructions.

      I do have an issue with my large Stihl bushcutter, though: it will not start again if I stop it after it's got warm...Any ideas? Nick

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      Eugene Brennan 15 months ago from Ireland

      Thanks Chris, I agree dismantling should be a less resort. It requires discipline and care if you don't wan't to end up with "spare parts" left over and no where to put them!

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      Chris 15 months ago

      Really useful article. Just gone through a week long learning process for servicing my hedge trimmer. I think taking things apart should be a last resort.

      My engine was cutting out when pressing the throttle for higher revs.

      I bought carb cleaner. Sea foam and a new spark plug.

      emptied of fuel. Pressed primer to remove even more.

      1. Cleaned air filter as per ur instructions.

      2 sprayed carb inlet with lots of carb cleaner. Pointing upwards and a little bit at a time and turned it over to suck it through. Left for 10 minutes. Turned over engine, will fire and run whilst it fires.

      3. Did same for sea foam then removed spark plug and filled cylinder with sea foam. Put back spark plug and leave for one hour. Emptied, then replaced fuel. replaced air filter. Pulled over several times. Then ran smokey for a while, but idling perfectly and revs fine.

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      Eugene Brennan 16 months ago from Ireland

      Hi Stuart, glad it worked out ok!

      If anything is loose on an engine, the problem can worsen over time as vibration shakes fasteners, and nuts/bolts can totally undo themselves. Star washers should prevent loosening, but unlike the case with lock nuts, once the head of a bolt becomes loose, a star or spring washer won't work. The hex socket head screws on your trimmer may have been buried in a recess without any washers so they could have loosened themselves. Loctite Threadlock or similar is useful for preventing this sort of thing happening.

      Anyway thanks for sharing this, it's just one more thing which others can check out when troubleshooting their machine.

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      Stuart Rodger 16 months ago

      On further investigation I saw that the carb just sits on top of the manifold and is sandwiched by a gasket when the air inlet and choke plate are bolted on. When I lifted off the carb assembly I could access the manifold bolts and found that one was almost completely loos and the other was not even hand tight. After tightening and re-assembly I found the engine starts and runs fine. Clearly the bolts had not been properly tightened at the factory! Another issue I found though when rebuilding, is that the markings for choke open and closed are the wrong way round ! So when I thought I was starting on choke I actually had choke fully open and when closing choke on hot engine it was stalling. This had also been contributing to the difficult starting even before the manifold bolts came loose.

      Now I know to operate back to front (;-)

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      Stuart Rodger 16 months ago

      Hi Eugene,

      I made the mistake of buying a cheap Chinese Imported Brushcutter Strimmer Chainsaw combination.

      It was very difficult to start first time, but did work in strimmer mode.

      When I changed over to brushcutter it starts with much perseverance but stalls on full throttle and will not start again. I have checked everything on your list, and when I put some neat petrol in the spark plug hole, it starts but then quickly stalls after about 30 seconds, so just burning the fuel I have added. I have stripped the air filter off the carb and cleaned out some leaf particles but generally looked ok. I notice however that the inlet behind carb into engine block, which is plastic secured by two socket head capscrews is loose. How do I get to this to tighten, without removing and dismantling carb? I think it may be sucking air into cylinder at this connection ?

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      Eugene Brennan 16 months ago from Ireland

      Hi Paul, I've been thinking about this and trying to envisage what scenario would cause this issue. If you read and followed the details above, you will know that when the primer bulb is released, fuel is sucked up around the edges of the red valve into the bulb. When the bulb is pressed, fuel leaves the bulb through the centre of the valve and returns to the tank. Meanwhile, the carburetor can "tap into" this fuel (on the intake side of the valve) and take a feed through the screen filter. Possibly the edges of the red valves are not sealing properly so that when the bulb is pushed, fuel is forced back into the intake line (the one with the filter). Because the line has a filter on the end of it, this would restrict the flow back, pressure would rise in the line and fuel could take a shortcut into the carburetor and flow through all the passageways, into the "reservoir pocket" as I described it and then out via the jet into the venturi. If the tank is pressurising because the vent valve isn't working properly (the one in the cap or tank body), this would compound the problem. Try loosening the fuel tank cap and see if it makes any difference and then take a look at the valve in the primer to see whether there is anything gumming it up or stopping it from sealing.

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      Paul 16 months ago

      Hi, I have an issue with my trimmer. It will not start. When I prime it for starting, fuel immediately comes out of my air filter. I have not taken apart the carb yet, I'm assuming something is clogged. Could the return fuel line be causing this?

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      Mario Rincon 16 months ago

      Thanks so much!

      You can take my comments off if you want now.

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      Eugene Brennan 16 months ago from Ireland

      Have a look at this eBay link: http://ebay.to/1XdDva4

      If you search on eBay for "string trimmer check valve", there are lots more like it.

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      Mario Rincon 16 months ago

      Hello Mr. Brennan.

      Thanks for answering my comment. I'm looking for the check valve of the carburetor (the red piece) to be specific. I know there are more like this out there, but I just wanted to know where I can find exactly this one. If you do know, please let me know to my email if you still have it. If not, I'll monitor the comment section to hear back from you. After that, you can delete your response once I confirm that I saw it.

      Thank you.

      This red piece is for my summer intern project by the way.

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      Eugene Brennan 16 months ago from Ireland

      Hi Mario! Unfortunately this was a trimmer which was badged/branded by a homestore. It was manufactured in China but the store won't give me any info on who their supplier is.

      If you're referring to using a part from the carburetor, take a look on eBay. There are lots of very similar looking carburetors to the one which is used on this trimmer.

      I'll have to delete your comment because I don't think comments can display personal information such as emails.

      Hope this helps!

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      Mario Rincon 16 months ago

      Hello Mr. Brennan...

      I just wanted to ask you the name of the trimmer (brand, model, etc.) that you used during this article.

      Thank you.

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      Eugene Brennan 16 months ago from Ireland

      Hi Ben, thanks for the comment.

      The consensus seems to be that pressure should ideally be in the 90 to 120 PSI range. However some people reckon that pressure can be as low as 70 PSI. Here's a link where compression values are discussed: http://bit.ly/1sQeCoc

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      Ben Lewin 16 months ago

      Thank you for your informative page.

      what should be the compression PSI for the string trimmer?

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      Eugene Brennan 17 months ago from Ireland

      Hello again Jan! So everything worked out fine in the end. Thanks for letting me know the outcome!

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      Jan Nienaber 17 months ago

      Hallo Eugene. Got my chainsaw back. Remote primer bulb replaced works like a bom. Thank you for your advise. Take care

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      Jan Nienaber 17 months ago

      Thank you sir. I given it back for repairs. Will let you know

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      Eugene Brennan 17 months ago from Ireland

      No problem! The engine could also be seriously flooded if you pulled the starter cord loads of times. If you haven't done so, leave it for about 10 minutes before reattempting a start.

      P.S. Just remembered something. I'm thinking trimmer but I forgot you have a chainsaw. Usually a chainsaw is started with the chain brake engaged and no throttle, not full throttle as I stated earlier. If the engine has a decompression valve, it should be opened. However, your chainsaw may have an automatic decompression valve, or none at all. Checkout the start procedure for a Stihl chainsaw which has full choke, half choke and run positions built onto into control lever. Your machine may be somewhat different though.

      http://www.stihl.co.uk/step-by-step-starting.aspx#

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      Jan 17 months ago

      Yes i did it worked in the beginning. I think it is a airlock or a blockage somewhere. I will take it in and will let you know. Thank you Eugene i appreciate your help.

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      Eugene Brennan 17 months ago from Ireland

      Have you checked for a spark? Did you use the recommended amount of two stroke oil in the mix? Squeeze the throttle trigger fully when starting and make sure the choke is on. Sounds like you may have a dud machine though because it should at least give a kick. It's probably best not to mess about too much with the trimmer if you're going to take it back.

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      Jan Nienaber South Africa 17 months ago

      Wow thank you for your response. Yes the primer bulb sit on the tank not on the carb. I can hear the primer action in the tank and if i open the cap i note bubbles. No fuel flows trought the carb. You may prime a 100 times ....nothing. If i pull the inlet line off the carb fuel actually burst out of the pipe at pressure. I rather should take it back.

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      Eugene Brennan 17 months ago from Ireland

      High Jan. If it's new and still on warranty, make sure to bring it back. If there are 3 fuel lines, you may have a remote primer. See the diagram above for an explanation on how the system works. The air intake is visible once you remove the air filter. When you say it has good fuel at pressure, what do you mean? If you try to prime the machine, fuel should flow back into the tank via the return line. Make sure when you prime that there is sufficient fuel in the tank so that air doesn't get sucked into the intake line. The result is that it can then take lots and lots of pulls of the starter cord to get the machine working

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      Jan Nienaber 17 months ago

      Hi Guys. I bought a wolf chainsaw china made. It work well for 2 days. From the outset it dit not prime properly. Now it doesnt prime at all. It has 3 pipe inlets at the carb. Will one be air? At the fuel inlet it has good fuel at pressure but it doent reach the outlet. It is brand new. I cant see ant damage to the pipes or the primer bulp. Could you help me. Thank you.

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      Eugene Brennan 17 months ago from Ireland

      Hi Matt. I don't think there were any model numbers or other marks on this carburetor, it was from an unbranded Chinese trimmer. Will check though. Are you sure this is the exact same carburetor? All these devices look very similar. For instance this one: http://ebay.to/1OlixxD which has a 10mm inlet bore. Don't know whether it would work though. The side which couples with the air filter is a little different. Also the hole spacing for the mounting bolts might not be the same. If you search for "trimmer carburetor" on eBay, you'll get lots of results and some products are generic, and not for any specific model.

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      Matt Man! 17 months ago

      Howdy, I need that exact carburator for my boat engine! I bought the engine from a coworker and he is no longer with my company. I can't locate a part number. Can you tell me? I can't believe you were working on the exact one!!! Please help. Thanks!

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      Eugene Brennan 17 months ago from Ireland

      There is some truth in that and an electric tool is also less likely to give trouble. 99% of the time it will start ok too. However there is the obvious disadvantage of a trailing power cord (unless you choose a battery model), and electric tools have less power for a given size than their gas counterparts.

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      Terry 17 months ago

      I am tired of fixing gas engine carburetors. I buy electric products anymore. Problem solved. Electric weed eater. Electric mower. Electric chain saw. Electric tiller (small one, still have a large gas one). Government regulations have totally screwed up small gas engine reliability with complication.

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      Eugene Brennan 18 months ago from Ireland

      This link shows the damage to a piston if you use neat petrol: https://www.youtube.com/watch?v=t001qQJ3ZtE

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      Eugene Brennan 18 months ago from Ireland

      Hi Nick, as regards damage, it probably depends on how long you ran the engine for, but if it turns ok, it's not seized. Forgetting the petrol shouldn't do any damage to the carburettor and even if the engine overheated due to lack of lubrication, the carburettor is separate from the engine block and shouldn't suffer heat damage. The engine may have overheated excessively which could have damaged the seal between the crankcase and engine block or crankcase cover and block (if block and crankcase are one piece), preventing crankcase compression. See the link at the top of this page which explains how a two stroke engine works. Some engines have a poppet intake valve which allows fuel into the crankcase and maybe this has gotten stuck or damaged from overheating. When you pull the start cord, is there much resistance? If it pulls much easier than before, maybe the cylinder/bore have worn, reducing compression. If you're adventurous, you could try opening the crankcase and investigate whether there is any damage to the valve. On my trimmer four screws hold an access cover to it (See photo with caption "Remove the cover over the exhaust outlet"). However before you do any of this, make sure the engine is priming ok by checking that fuel returns to the tank when you push the primer bulb several times and follow the proper startup procedure.

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      Nick 18 months ago

      Hi,

      If you're a forgetful idiot and you forget to add the oil to the petrol.....run the engine and then it dies what would be the likely damage caused ?

      The engine still turns so I don't believe I have frozen the engine....I have now added the correct oil mix to the petrol.....I have spark....I have tried putting a thimble of petrol into the sparkplug chamber but not starting....

      I checked the exhaust system and cleaned it.....

      When it turns over sometimes smoke comes out of the exhaust

      I now have the carb off and apart at the moment....terrified as I've never attempted anything like this before.....

      Would I have caused terrible damage ?

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      Eugene Brennan 18 months ago from Ireland

      Hi Jean Louis, What is the model/make of the trimmer? I might be able to source a manual. However, if the machine is running ok, its better not to fiddle too much with the screw. Also if the high speed mixture is too lean, it's possible to exceed the speed rating of the engine, causing overheating and damage. Sometimes there is a "H" or "L" marking near the screws, indicating they are for adjusting the high and low speed respectively. As a starting point, screws should be screwed in gently until the hit the bottom of their travel (don't screw tight to avoid damage). Next unscrew them one complete turn. This is the base point. Run the engine at full throttle for a couple of minutes until it is warmed up. To adjust the high screw, run the engine at full throttle and turn the screw clockwise until the engine runs poorly. Next turn it anti-clockwise until it begins to run erratically again. Set the screw a little to the left of midway between these two limits. To adjust the "L" screw, allow the engine to idle and follow the same procedure as above, setting the screw about 1/8 turn anti-clockwise of the midway position.

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      Jean-Louis 18 months ago

      10 months ago Jan/Prague/Czech spokes about another needle/screw not covered by this tutorial. It's going through the pivot screw in the accelerator lever (black plastic part) on the carb and through the rotating barrel-ous part inside the carb body, and usually, it's sealed. ...... Do you happen to have an idea what should be the basic adjustment point for that particular needle?

      Your responses seems to be out of concern; you don't relate to the screw of Jan but to the idle screw

      Can you give us more information about that needle screw that is located axially in the barrel, only screwable by a tiny screw driver through the central hole of the yellow screw used as rotation shaft of the command lever ( just below the end of the screw driver on your above picture "Remove the end of the throttle cable".

      May be, this "needle-screw" is used to adjust fuel/air mix at middle run.

      If possible, please precise the basic adjustment point for that particular needle.

      Many thanks in advance!