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

Updated on November 13, 2019
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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.

Weed Eater Won't Start?

String trimmers 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).

If you found this guide was useful and it helped resolve the issue with your trimmer, please take the time to share it on social media, Facebook, Pinterest etc.

Typical gas string trimmer
Typical gas string trimmer | Source

Other Names for a String Trimmer

These machines are generally called a string trimmer in the US. Other names are:

  • Weed eaters (brand name)
  • Strimmers (Ireland and UK)
  • Line trimmers
  • Weed whackers
  • Whipper snippers (Australia)

Common Reasons Why a String Trimmer Won't Start

  • A dirty or cracked spark plug
  • Faulty magneto
  • 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.

How a Two Stroke Engine Works

A two stroke engine as it goes through it's cycles.
A two stroke engine as it goes through it's cycles.
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 = 473ml

1 US gallon = 3.79 litres

1 UK imperial gallon = 4.54 litres

1 UK gallon = 1.2 US gallons

1 UK imperial pint = 20 UK fluid ounces = 568ml

Priming the Carburetor on a Two Stroke Engine

  • 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. This is an important step because it fills a small reservoir in the carburetor with fuel, which the trimmer draws on during use.
  • 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 a Trimmer, Hedge Cutter, or Chain Saw Properly

  1. Rest the machine on the ground, clear of any debris.
  2. If the engine has a separate on/off switch, move it to the on position, otherwise move the choke control from the off position to the full choke position
  3. Prime the engine as described above
  4. Pull the starter cord and if the engine doesn't attempt to start after a few pulls, try re-priming.
  5. Once the engine runs, wait until it cuts out or for about ten seconds, whichever comes first. If it only gives a "kick", try pulling the cord again
  6. Then turn off the choke and allow the engine to continue running
  7. If the engine won't start, repeat the process from the start by turning the choke back on, priming the engine, and pull-starting.
  8. 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.
  9. 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.

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.

Check for Cracked Fuel Lines

Leaking lines are bad news, not just because they result in a pool of gas on your garage floor, but they may also stop fuel being properly sucked into the engine (air bubbles end up in the line) causing an engine to stall or even difficult starting. Splits/cracks in lines mightn't be obvious. On this machine when I put it away after doing some cutting, even if the tank was on its back and fuel lower than the sealing bung where lines entered the tank and also lower than the tank cap, it still dripped. While doing some maintenance, I discovered the fuel lines were broken half way through, inside the seal where they passed through the tank.

These fuel lines split where they passed through the tank seal.
These fuel lines split where they passed through the tank seal. | 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!)

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
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).

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.

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.

If you've checked all the easy things to remedy above, you may need to checkout the carburetor:

  • Diaphragm/pump section in carburetor may have stiffened so it can't pump/meter fuel
  • The jet could be gummed up and dirty

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 metering needle or needle 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 from 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. | Source

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 of the "umbrella".
One-way or check valve. Check there's no dirt stuck in the "duck bill" valve section in the center of the "umbrella". | 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

How a Diaphragm Works 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 when the engine is running on full throttle external atmospheric pressure acting on the back of the diaphragm pushes the centre of it inwards. This forces a metering lever/rocker arm to pivot, lifting the metering needle/needle valve up out of its seat, allowing fuel to be pumped into the reservoir pocket
  • When the engine returns to an idling state there's less demand for fuel, so the reservoir starts to fill pushing the diaphragm out. The centre of the diaphragm no longer pushes down on the lever and the needle valve starts to close, preventing further fuel from being pumped in. So the reservoir 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

A Short Video Explaining How a Two Stroke Diaphragm Works

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 and fuel passageways 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. Using IPA to clean these parts and the metering needle might be a safer option to avoid damage.

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
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. You then need to either replace the switch if it's faulty, or reconnect either of the two wires connected to the switch. Usually they break at the points where the wire is terminated by spade clips either at the engine or switch.

Winter Storage of Two Stroke Engines at the End of the Season

To avoid problems next year, drain all fuel from the tank and run the engine until it cuts out. Leaving fuel in the carburetor over winter or when a tool isn't going to be used for a long period can cause everything to stick together, resulting in difficult starting.

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

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This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

  • How do I check the oil level in an engine?

    2-stroke engines don't have a sump (reservoir) containing oil.

    If the trimmer is 4-stroke, you can follow the procedure outlined in Jacks Small Engines here, or check your manual for the procedure for your specific model. If you can give me the model number, I can check the details.

    https://www.jackssmallengines.com/diy/how-to-check...

  • I have a 26cc Chinese hedge trimmer which refuses to start. All the usual checks I have carried out and still no joy. It has new fuel, new spark plug, new carb, and still not even a kick. There is plenty of spark, fuel and air. Is it possible that the spark timing has moved, and if so how does one check it?

    Yes, it is possible that the flywheel has moved on the shaft and affected timing. There may be a flywheel key which could have sheared allowing the flywheel to shift from the correct position. A spark should occur when the piston is near the top, dead center (TDC), so look into the spark plug hole and check whether the piston is near the top of the cylinder when the magneto coincides with the flywheel magnets. Another thing you should check is that the crankcase cover (if there is one) is sealing properly and the gasket is ok.

  • Why does my trimmer only run with the choke mostly closed?

    The air filter could be dirty, but it's more likely that something is restricting flow through the carburetor such as a blocked jet.

    When starting a trimmer, it can be reluctant to run without choke for the first 20 seconds or so until it warms up and get's going and the carburetor reservoir fills with fuel. So try alternating running it with full choke and no choke until it can run without stalling on no choke.

  • My string trimmer's starter rope has no tension on it. Has it disconnected from something?

    It sounds as if the recoil spring has broken or become detached.

    Have a look at this guide for information on how to replace one:

    https://dengarden.com/home-improvement/Stihl-KM-56...

  • Can the exhaust valve not open on a Mantis tiller?

    4 stroke engines have exhaust valves which can possibly stick. 2 stroke engines don't have exhaust valves.

© 2014 Eugene Brennan

Do you have any questions or comments to add?

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    • eugbug profile imageAUTHOR

      Eugene Brennan 

      2 weeks ago from Ireland

      I find sometimes the engine dies if I open up the choke too fast. So setting it half way while continuing to run the engine is best. Same goes for opening the throttle too quickly (although this can be due to the diaphragm /pump membranes getting old and not being able to feed fuel fast enough to the jet on throttle up).

    • profile image

      Wallace Tom 

      2 weeks ago

      Found the answer to my question. It is the same as your suggestion. Throttle off and choke fully closed, then go to half choke with throttle open. Engine ran on first pull set on Half Choke then proceeded to Run. Thank you for the answer to my carb problem. It was starting to get frustrating.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      2 weeks ago from Ireland

      Maybe try priming more (push the button well in 10 times). Start with throttle off and choke full on. Ease throttle on slowly once you turn off choke (or maybe leave choke half on until engine runs well).

    • profile image

      wallace tom 

      2 weeks ago

      My question concerns a Homelite 96SS (UT32651) string trimmer. It will only start if I pour a little cap full of pre-mixed fuel into the carburetor. It runs fine after that. I replaced the carb with a new unit, new fuel tank, fuel filter, fuel lines , fuel cap. Cleaned spark aresstor, checked for scoring on piston both intake and exhaust look good. Good Compression, New carb gaskets

    • profile image

      mikealpha 

      2 weeks ago

      Finally got the tiller fixed. After replacing the cylinder, piston, coil, the carb, and fuel lines, come to find out that the coil, I ordered on ebay was the wrong one, (right one ordered, wrong one sent) even though the sparkplug tested good with a spark tester.

      Note for readers here the Stihl coil will have designated timing in their coil modules. This 12 year old Stihl tiller runs really well once again. Woo hoo!

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      2 weeks ago from Ireland

      This can be caused by a blocked up exhaust port/muffler or alternatively the cylinder is flooded and filled with fuel that won't compress with the plug in. Turn it upside down to drain any fuel and check the exhaust also. Larger engines have decompression valves and if these aren't opening properly to partially vent the cylinder, this can make it difficult to pull the cord.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      4 weeks ago from Ireland

      Thanks Dale for the latest "instalment" of the story"

    • profile image

      Dale 

      4 weeks ago

      Hi Eugene,

      Report time on my previous message.

      I tried cleaning the carb out but found small bits of rubber floating around.

      The first idea was to by a new gasket set but Ebay have new complete carbs for just £6 so one bought one as its actually about the same price.

      I also checked for spark and it was very bright.

      This morning the carb arrived so I fitted it, primed 5 times and.....nothing,

      At this point I was about to just put it back in the shed but instead I pulled the plug and found it was soaked with fuel.

      A quick visit to the gas cooker and no priming with the choke set to run and bingo it runs.

      It was a bit smokey and smelly at first but that has now cleared and it starts first time.

      Thanks again for you great article as this strimmer would have been destined for the scrap pile otherwise,

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      6 weeks ago from Ireland

      Hi Dale,

      Thanks for that!

      I find with my hedge trimmer and strimmer which are 20 years and 10 years old respectively that they can die on throttle up after initial starting. The jets aren't blocked in the carburetors, but I reckon over time, the pump section can get stretched and lose elasticity so that it doesn't pump so well. It's ok on idle but when there's more fuel demand, it just can't cope, well at least to get fuel into the fuel pocket/reservoir. That's why it's so important to prime well before starting (up to 10 presses). I usually increase throttle over a period of 10 seconds or so and then everything runs fine.

    • profile image

      Dale 

      6 weeks ago

      Hi Eugene Great article, I have had a Spear And Jackson SGT33N with strimmer, chainsaw and cultivator heads given to me.

      It did start and run with a quick squirt of carb cleaner after which it did start on petrol but now it wont start.

      Before it stopped starting on petrol it would not rev, my guess is a carb clockage because when I squirt carb cleaner in the carb it starts but wont keep running.

      It has only been used a couple of times but not used for a year or more so I will get a new spark plug, fuel lines and new fuel grommet where the lines pass into the fuel tank as this leaks too.

      I will report back soon, fingers crosses.

    • profile image

      Craig 

      7 weeks ago

      I. Got a. Stihl. Curve trimmer. I. Put. New. Lines. Use. Old. Filter cleaned. Fuel. Up. To top. Of. Line. Just. Before entering. Carb wont run. Only. When. You. Spay carb. Stuff. It runs. For. A. 2 sec. Or. Instantly.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      8 weeks ago from Ireland

      Many thanks Don for the feedback! If you think there are any errors, let me know.

    • profile image

      Don 

      8 weeks ago from Gelorup, Western Australia

      This is one of the most helpful articles on small engines I have come across. I hope you are getting some income from this.

      i am a mechanic by trade and can recognise your thorough knowledge and experience in this area.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      3 months ago from Ireland

      Check out the section "Trimmer Cuts Out on Full Throttle" above.

    • profile image

      Jason 

      3 months ago

      Weedearer crank wont stay running

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      4 months ago from Ireland

      It depends on the model of the strimmer and what the screw is actually for. An idle screw which sets the idle speed of the strimmer will be located close to where the throttle cable attaches onto the carb. It simply acts as a stop, preventing the speed from dropping below a certain rate. There may also be a high and possibly low speed mixture adjustment screw, usually indicated by "H" and "L". Stihl recommend the high speed screw is reversed out one turn after screwing it in (gently) until it will go no further. However it may depend on the strimmer. In any case here's Stihl's manual for a specific model of strimmer giving the adjustment procedure.:

      https://m.stihlusa.com/WebContent/CMSFileLibrary/I...

    • profile image

      paulWILIAMS 

      4 months ago

      HI I HAVE A STRIMMER BUT IT AS ONLY 1 SCREW ON CARB IF I TIGHTEN IT UP HOW MANY TURNS FOR IT TO B ALRIGHT STRIMMER NO GO

    • profile image

      Chris V Sexton 

      4 months ago

      My 4-stroke Ryobi will not start. There's a strong spark, but if I drop fuel directly into the cylinder it still doesnt run, or even fire briefly. I haven't yet checked the compression as I need to buy a tester. It seems okay though (i.e some resistence on starter string). Is it possible that the engine is worn out already (it is 2-years old)? Are there other things to try? It will be annoying to throw it out. Given how little it it has been used I reckon it's cost about £20 per use over two seasons. Not good! Thanks.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      5 months ago from Ireland

      Check your lines are the right way around by emptying the tank so the return line (without the filter) is above the surface of fuel and the other line is below. If the lines are correct, when you press the primer, fuel should flow out the line without the filter. It takes about 10 pushes of the button for air to clear out of the carburetor.

    • John Lfc profile image

      John Lfc 

      5 months ago

      done fix new fuel tube both and filter and tank full but seem air bubbles come thought the tube ?

    • profile image

      mikealpha 

      6 months ago

      yes on the fuel lines and made sure the arrow was pointing to the carb. It is sparking. I used an inline coil tester and it was glowing orange.

      The primer bulb is sucking up fuel and has the spongy feel to it.

      Oh I also removed the spark arrester screen in the exhaust and cleaned it.

      Thanks for the carb tuning link

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      6 months ago from Ireland

      I assume you replaced the fuel lines the right way around?

      Is it sparking ok?

      Is the primer button if it has one sucking up fuel and filling and feeling "spongy" and returning fuel to the tank?

      Have a look at this guide which gives info on tuning the carburetor.

      https://www.manualslib.com/manual/1193810/Stihl-Mm...

    • profile image

      mikealpha 

      6 months ago

      Hi I have a Stihl MM55 tiller. I have replaced the coil, the carb, the engine, the fuel filters, air filters, i've tested the coil with an inline tester, tried numerous sparkplugs, ohmed the start/stop switch, and even tried putting fuel and starter fluid in the carb, but this thing will not fire. My magneto gap is set at about 12 thousandths. I looened my fuel gap to see if that made a difference and it did not. The only thing I can think of is timing , but I just guessing. Its driving me nuts

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      6 months ago from Ireland

      Hi,

      I don't know the number of the air filter because the trimmer is a Chinese import, branded by a home store. The store won't tell me who their supplier is. There is a "00068" printed on it but that doesn't come up in a search. The only thing I would suggest is that you Google "trimmer air filter" and then click/tap on images and browse through the results to see if any of them look like the one you want.

    • profile image

      JohnnyJ 

      6 months ago

      Eugene... great article. I just got my line trimmer running like a champ, however the air cleaner cover is broken off... and the filter missing. Mine is a Craftsman, but the part I need is no longer available. It's a Briggs and Stratton part# 790567. I noticed your line trimmer's air filter also has an integrated choke, and is using the same carb mine is. What air filter part number is yours?? Can you find out if it's still available?? It'd sure be a life saver, as I'm sure it'd work just fine on my trimmer.

      Thanks a bunch.

    • profile image

      Dean1978 

      6 months ago

      The mechanism is working fine. 10 mins earlier i thought it was a carb problem as the throttle worked, but the engine stalled when the choke was closed or from revving to much.

      Many thanks

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      6 months ago from Ireland

      Is the cable working the throttle arrangement on the carburetor? If it is, chances are something has gone awry with the jet that lets fuel. Check first though that the cable is pulling the mechanism and causing it to pivot ok.

    • profile image

      Dean1978 

      6 months ago

      Hi Eugene

      I have a generic strimmer very similar to yours that will not throttle up when i press the trigger.

      Ive had to remove the saftey latch (in front of the saftey cut out switch) as it become stuck in position. The throttle cable seems to be working as it should but has no effect on the engine speed.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      7 months ago from Ireland

      Hi Nigel,

      Sounds as though you put an awful lot in, so it best to allow it to drain and evaporate. If you try this, you only need to use the equivalent of a teaspoon or so. Don't pour it in directly from the can. Maybe move around the engine at different angles to see if you can drain any more out and then try to pull start with no choke to see if any more flows out. Then follow the starting procedure and troubleshooting tips above.

    • profile image

      Nigel Cross 

      7 months ago

      My strimmer won’t start and when I pour petrol in to the plug hole, it comes out of the exhaust. Is it serious?

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      7 months ago from Ireland

      Whether the line attaches to the straight or bent connector may depend on the carb model. The straight connector on mine attaches to the fuel intake line with the filter, but you can check you have the lines the right way around by pulling up the return line above the fuel level in the tank. If everything is connected properly, when you press the primer button, fuel should return via the line without the filter. If the lines are the wrong way, bubbles of air will eventually come out of the filter after pressing the button several times.

      As regards adjusting mixture screws, there may be one to three screws for low speed mixture, high speed mixture and idle speed, depending on the manufacturer/model of the trimmer. Stihl recommend both L and H screws are opened 1 turn after turning them in very gently (to avoid damage to the jet tips and valve seat) and idle speed needs to be set also. (see page 23 here)

      https://m.stihlusa.com/WebContent/CMSFileLibrary/I...

      The procedure may be different with your trimmer so ideally you should source the manual and follow it.

    • profile image

      Mick 

      7 months ago

      I have put a new Carby, fuel lines, filter & spark plug on my whipper snipper. Unfortunately I moved the adjustment screw & are now not sure where to put it to try & start my machine. I have tried to start it & it has started on choke but only runs for less than 10 seconds. I take the choke off like I used to, when it would then fire up no problems, but now it won't kick at all with the choke off. I adjust the screw in & out to get it to idle longer but it won't. One question about the fuel line that goes to a filter, my Carby looks exactly as the one you have & I understand the way to check which is which that you explained but I am I right that the line that my filter is on attaches to the straight connector not the bent one. Any tips will be appreciated.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      7 months ago from Ireland

      Hi Malesa,

      Have you read the article above?

      Basically you need to try all the things outlined.

      Use new fuel.

      Use the correct proportion of two-stroke oil.

      Check spark.

      Make sure the primer is sucking fuel in ok.

      Check the vent in the cap or tank.

      If the trimmer runs out of fuel during use, it'll need to be re-primed by pressing the primer button about 10 times. Usually when the button feels spongy, sufficient has been sucked up.

      Start with choke on but leave off choke if a warm engine is being re-started.

    • profile image

      Milesa petrovic 

      7 months ago

      Hi Eugene Brennan , I have Royal einhell MSB 34 trimmer .and wan't start again . I could not repair . Please help . Thanks

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      9 months ago from Ireland

      Hi Tom, could be leaking gaskets or diaphragm. /the other possibility is that the needle valve and/or or seat in the carburetor isn't sealing properly. Sometimes if you try pulling the starter cord repeatedly and the engine doesn't start, the carburetor can flood and a little fuel will drain out when you put the engine on its side after removing the air filter.

      Check too that fuel isn't actually coming from a split fuel line.

    • profile image

      Tom Tefau 

      9 months ago

      hii Eugene Brennan, i have a problem with my homelite trimmer, fuel keep coming from air filter after priming, even when i pull start it its going worst by leaking, any suggestion ?thanks

    • profile image

      Charl 

      10 months ago

      Thanks very much very helpful

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      10 months ago from Ireland

      Hi Alen. A couple of things to check. Make sure the tank vent located in the tank cap or on the body of the tank isn't blocked. This lets air flow into the tank as fuel leaves it, otherwise it wouldn't be possible for the carburetor to suck fuel out of the tank. Another scenario is that vapour can build up in the tank when in use due to movement and heat and when the machine is stopped for a while, this can condense, causing a partial vacuum if the vent doesn't work, preventing the pump in the carburetor from sucking in fuel. Try releasing the cap and tighten again and see if that gives a result. if it does, possibly the vent is clogged. Maybe also the plug could have sooted up .Clean it with a brass brush. Apparently a steel wire brush is supposed to scratch the electrodes. Not sure whether this is really the case.

    • profile image

      Alen 

      10 months ago

      Hi Eugene,

      I have problem with warm start of trimer Stihl FS 450 even without using switching on decompression gumb- it does not start smoothly like my previously used older version of stihl trimer FS 160. I have replaced air and fuel filters, spark plug but problem remains unsloved. Could you advice what to check the next?

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      10 months ago from Ireland

      Sounds like a problem with fuel flow.

      Check the carburetor isn't loose.

      Check primer sucks up fuel and returns it to tank. There may be air in the line so try priming again maybe ten times to clear any air.

      If you pull too many times, the carburetor can flood, so remove the filter and turn it on it's side to drain out any fuel that's accumulated.

      Mixture should be 50:1 or 25:1, whatever's recommended on the oil or engine manual.

      When and if the engine starts, let it run for about 5 seconds before turning choke off then try starting again with choke still off and if it doesn't start, turn choke back on and retry. If all that doesn't work, you may have to dismantle the carburetor and clean any blockages.

    • profile image

      Jay 

      10 months ago

      My strimmer doesn't start. Plug is new, fuel pipe is new, fuel mix is new, but not starting. So took out the spark plug, and poured some fuel into the hole, and put the plug back on, and started. Engine started as if it will run, but it only ran for 2-3 seconds, and dies. And it will not start no matter how many times pull.

      Fuel in the spark plug hole poured, and it starts and runs for 2-3 seconds with smoke, then dies. Where should I look into now?

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      11 months ago from Ireland

      Sounds as though the fuel pickup isn't staying under the fuel when you tilt the the engine. If there isn't much fuel in the tank, this can happen, but usually the fuel line with the filter on the end can move around so that it's always submerged. If the line has become stiff and rigid, this may not be happening. Alternatively the fuel tank cap retainer could be catching in the line and stopping it moving. Try moving the trimmer around with no fuel in it and see if the line is able to move. Even turning the trimmer at an angle for a short period of time and then reorienting it can cause a stall as air gets into the line (prime to flush the air out).

      It's best to keep your tank always at least full to avoid this problem.

    • profile image

      11 months ago

      Hi i have a still edge trimmer,.it starts and runs but stalls when i have the trimer on any angle or tilt, any ideas

      Cheers

    • profile image

      ryan 

      11 months ago

      my wheateater didnt wanna spin.

      after taking it apart and cleaning it, it spun but the wrong way.

      Can you help me?

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      13 months ago from Ireland

      If you mean the recoil starter pulley, it may be jammed somehow by the cord, spring or ratchet pawls. Best thing to do is remove it from the trimmer and see if it is actually jammed or its the flywheel that is stuck.

    • profile image

      Gary abdul 

      13 months ago

      How to fix the pully which stuck if i want to start. Looks jammed. What can i do.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      14 months ago from Ireland

      Hi Paul,

      Sounds like a fuel flow problem. Enough fuel is getting to the engine for it to idle, but when the throttle is opened up, not enough sprays into the airway.

      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.

      The jet in the carburetor or metering needle/seat may be clogged.

    • profile image

      Paul 

      14 months ago

      Strimmer starts no problem but when I increase the revs it cuts out. any ideas

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      15 months ago from Ireland

      I don't know. We're always advised to use genuine parts when replacing. There's a possibility that gaskets may not match the fuel entry port on the manifold or the passageway to the crankcase used for operating the fuel pump in the carburetor. Also too much fuel could be metered into the engine because the carburetor is designed for a more powerful machine, so it could over speed.

      Often power equipment sold by homestores or tools distributors is made in China and just badged. So machines with different brands are the same and use the same parts.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      17 months ago from Ireland

      Hi John,

      Sounds as though the shaft is broken or the square end of the shaft is worn and the corners are rounded so it doesn't turn the head.

      Have a look at this video.

      https://www.youtube.com/watch?v=ZHFMSeg8Rwg

    • profile image

      John Harrod 

      17 months ago

      My Ryobi weed - eater engine starts and sounds fine but the spinner seems to be frozen and won't turn

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      18 months ago from Ireland

      Check your air filter is clean, also remove any carbon deposits from the muffler, use new fuel and make sure you have a spark

    • profile image

      Sheik 

      18 months ago

      How do you check if you're getting a weak suction in the carburetor .

      I think the air /gas is not been pulled in enough.

      Is there a leak somewhere?

      Brand new carburetor, gaskets and new air filter.....compression 120.

      Please help

      It's a craftsman blower

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      18 months ago from Ireland

      Hi Gabriel,

      I've never come across this problem, but the consensus seems to be that an air leak can cause the problem (or cracks in the fuel lines, but you've replaced those).

      Have a look at this discussion:

      https://www.arboristsite.com/community/threads/sti...

    • profile image

      gabriel 

      18 months ago

      I have a Craftsman 32cc 2stroke. I had to change fuel lines and clean out the carb. Now after Getting it to start It revs up high and shuts off. Then it wont start back up for a few minutes. I was told this can be a symptom of a bad gasket/ air leak from the carb to the motor? What do you think?

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      18 months ago from Ireland

      You're welcome Tom, Let me know how you get on and it'll contribute to the knowledge base. You could also try getting a carb kit as an alternative to a new carb.

    • profile image

      Tom Stenger 

      18 months ago from Delaware, Ohio

      Thanks for your help Eugene :^). I'll take it apart (carefully) and check those things out. Ma opt to purchase a new carb if needed.

      Much appreciated!

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      18 months ago from Ireland

      Temperature can drop substantially as gas expands, hence the vapour trails on the wings of aircraft in low pressure regions of the wing, or you'll notice how an aerosol can gets really cold as spray is released. Another example is when you blow out air from your mouth onto you hand. If your mouth is wide open it feels warm, but if you blow out with your lips tightly closed, the air feels cold, because the temperature drops as it expands on exit. Moisture will also condense when air is compressed because it can only hold a certain mass of water per volume at a specific temperature. So this is why water condenses in the tank of an air compressor, as molecules of H2O get squashed together when the air is compressed.

    • profile image

      Grullafo 

      18 months ago

      Yes. I did notice that the water seemed to condense as it hit the surrounding air. Lots of little droplets that then coagulated into a stream of drips. The outside temperature was in the high 20s, but the humidity was extremely high, enough to trigger hundreds of thunderstorms over England and Wales. I've never seen a 2-stroke do that before!

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      18 months ago from Ireland

      It's unlikely to be water because the heat of the exhaust would instantly evaporate it. A by-product of combustion is water vapour, but you normally only see this on frosty days from e.g. a vehicle exhaust or condenser boiler.

      You may get black/brown tarry creosote coming from the exhaust but this is normally a smelly sticky substance. I would have thought excess fuel would combust, so on second thoughts maybe as the exhaust gasses expand, the temperature drops to a degree that water vapour condenses out in the humid atmosphere.

    • profile image

      Grullafo 

      18 months ago

      I have an old Mitox 2-stroke strimmer. I used it yesterday to cut down long grass using the thickest nylon cord that I could find. The engine was worked hard, and after a few minutes I noticed large quantities of fluid coming from it. At first I thought that the petrol tank had split, but on further investigation it turned out to be dirty water coming from the exhaust. The weather was very warm and humid. Is this normal?

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      18 months ago from Ireland

      Hi Tom,

      possible causes are a stiff diaphragm that pushes downwards and keeps the needle open, a needle stuck open or not sealing, or a choke plate not opening properly.

    • profile image

      Tom Stenger 

      18 months ago from Delaware, Ohio

      Hi Don,

      Thanks for your tutorial and all the info on 2 Cycles. It helped me walk through possible issues with my Stihl MM55. It's run fine the past4 or so years we had it. We're in Ohio and I cleaned the gas and everything out , successfully starting it before loaning to my friend for the day, who ran it successully.

      I then tried less than a week after to start it...wouldnt start so I emptied the mixture out and replaced with mine (Still specs).

      When I crank it, it's spitting gas mixture out of the carb in rhythm with my starter-pullks. The first couple times I tried starting, it had tried to start, then this. Not blaming my friend at all, but is it a broken diaphragm, broken piston? I can probably pick up a matching carb at our Ace /Stihl dealer up the road.

      Any ideas suggestion much appreciated!

      Tom

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      18 months ago from Ireland

      I haven't been able to find that kit Don, the cheapest carburetor is $21.27 here:

      https://www.partstree.com/parts/mtd/parts/753-0851...

      I've posted on a forum and asked if anyone knows whether it can be sourced.

    • profile image

      Don 

      18 months ago

      Thanks for the quick response. The blower model is a craftsman 41BS2BVG799 27CC engine. I can find a replacement for the entire carb which is model MTD 753-08517. But why replace the carb if the only issue is the diaphragm.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      18 months ago from Ireland

      Hi Don,

      Do you have a model number for the blower?

      It's not recommended to use non genuine parts, but a lot of the time, origianal parts aren't available. You could try and see if they work. A part may feed too much fuel into the engine and since thes small engines don't have a governor, it could possibly over rev.

    • profile image

      Don 

      18 months ago

      I have a Craftsman Blower and it looks like the diaphragm needs to be replaced. I can not find a rebuild kit for the model number of the blower. I do see other kits where the diaphragm looks the same but they do not list my blower as a compatible model and some of the kits that look like they will work are for chainsaws or trimmers. Could I use a kit that the diaphragm looks the same or are there really differences between diaphragms?

    • profile image

      Arwel Richards 

      18 months ago

      Great article - many thanks

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      18 months ago from Ireland

      If the strimmer has a clutch, it may be broken or the clutch shoes worn. Otherwise there's some problem down along line where the engine couples to the flexible drive shaft and head, again due to wear or something snapped.

    • profile image

      andrew 

      18 months ago

      strimmer is working ( motor) but not the strimmer head not turning

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      18 months ago from Ireland

      Hi Calvin,

      If the strimmer is new, I would advise contacting the retailer/manufacturer as you'll probably invalidate warranty if you start meddling with it.

      Difficulty in pull starting can be due to a partially blocked exhaust due to combustion residues or possibly reed valves not opening properly.

    • profile image

      Calvin 

      18 months ago

      Hi, I'm also having the same problem but with a new strimmer (Hyundai HYBC5200). Worked for 30 minutes then I couldn't start it again, too much resistance from the pull cord. When the spark plug was removed the resistance was gone. Plug was dry, checked gap which was fine and replaced spark plug anyway but as soon as it's in the cord won't pull?

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      19 months ago from Ireland

      It depends on the trimmer. As you can see in the photos above, the check valve under the primer bulb is exposed by undoing a plate held on by four screws. You should then be able to grip the flappy edges of the valve and pull it out gently at an angle. The check valve in the tank or tank cap differs a lot between each models so I can't help you with that.

    • profile image

      Steve 

      19 months ago

      How do I remove the check valve

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      19 months ago from Ireland

      Hi John,

      When you say won't pull do you mean not start or there's resistance to the cord being pulled? If there's a decompression valve it could be faulty and staying shut when you try to open it. This engine also has reed valves on lead air intake ports so maybe these are staying closed. Does the recoil starter turn really freely when the plug is removed?

    • profile image

      John Blevins 

      19 months ago

      I'm trying to figure out why my trimmer won't pull? I had replaced primer bulb and now it won't hardly pull at all unless I take spark plug out then it pulls and recoil just fine. My trimmer is a redmax gz25n please help

    • profile image

      Chad 

      19 months ago

      This is a very thorough post! Very nice explaining the pathway of the gas in the carb too.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      19 months ago from Ireland

      Not the version I have. It's designed to supply the correct percentage of fuel in the mixture that feeds the engine.

    • Peter Heaslip profile image

      Peter 

      19 months ago from Australia

      Thanks eugine,do you know if they can be adjusted or there just set as is.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      19 months ago from Ireland

      Hi Peter, there's no adjustment on mine except for a screw that sets the idling speed of the engine.

    • profile image

      peter 

      19 months ago

      Hi eugine,i have the same carby as the one your showing,(we call the chinese throw aways) as they dont have hi/lo adjustment screws,can they be adjusted or are they really throw aways,thanks peter.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      20 months ago from Ireland

      Check it's priming ok by putting a small amount of petrol into the tank. Then push the primer button and make sure fuel returns to the tank via the line without the filter. Dismantling the carburetor is a fiddly business. So maybe check the seal on the crankcase is ok first, because the crankcase needs to hold vapor before the back of the piston forces it up into the cylinder. On my strimmer, the crankcase cover is held on by 4 screws (see photo, bottom right where I'm removing the cover over the exhaust screen) and the seal is a rubber gasket that runs all around the perimeter. You could check if that's ok, but if it's difficult to remove the screws, don't force them.

    • profile image

      Henry Lobb 

      20 months ago

      Hi Eugene

      It wasn’t a screeching noise or anything that sounded like rubbing...more like an over-revving noise if anything. When I say over-revving, I mean more revs than usual when idling, rather than beyond the normal level at full throttle. The engine turns OK when I pull the cord: there doesn’t seem to be any resistance. It just doesn’t fire into action.

      Best

      Henry

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      20 months ago from Ireland

      Hi Henry,

      That sounds odd? Was it a screeching noise as though something was rubbing? The flywheel could be rubbing the magneto coil if it was loose, or something else along the drive shaft. Does the engine turn ok without any resistance when the cord is pulled? It could also be something potentially fatal like a seized crankshaft connecting rod "bearing".

    • profile image

      Henry Lobb 

      20 months ago

      Hi Eugene, thanks for the excellent article. Before I go taking things apart that I might not be able to get back together I was wondering if you might give me a likely diagnosis of my strimmer's problem when I say that the engine note went quite high as I was using it and then it lost power and cut out. Since then I've not been able to start it...the engine did "kick" a few times initially as I attempted to start it, but since then it's been totally unresponsive. Air filter and fuel lines look fine...I don't (yet) have a spark wrench so I've not been able to test the spark plug. Any thoughts much appreciated!

    • profile image

      Martin 

      20 months ago

      Excellent explanatory article. Came in very handy to understand my brushcutter and fixed it afterwards.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      22 months ago from Ireland

      Check the fuel lines to make sure they aren't punctured. Check the primer sucks up fuel and returns it to the tank when pressed. If that doesn't work, you'll have to open it up an see if there are any blockages or whether anything has gummed up.

    • profile image

      Jakw 

      22 months ago

      I have a small mini Mac 120 chainsaw engine that has a carberator that will not suck fuel. I changed all the gaskets in it and the bulb seems to be good. There seems to be good compression when I pour a little gas into the cylinder. Anyone know what’s going on?

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      22 months ago from Ireland

      It could be a problem with the ignition module/circuitry. Sometimes if these are faulty and they get hot, expansion causes a connection to go open circuit. The problem rectifies on cooling.

    • profile image

      Ian 

      22 months ago

      Eugene thank you for the advice. The problem is when it cuts out, it takes awhile before I can start it again.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      22 months ago from Ireland

      Hi Ian,

      Usually this is an issue with fuel flow, but read the section on starting an engine properly above because often, starting a two stroke engine involves switching between full choke and half choke until the engine settles down. Also make sure you prime the engine about ten times before starting.

      If the engine starts to struggle before cutting out, turn the choke half on and see if it recovers and let it run for a while. If it starts to struggle again and splutters, this is likely because it is starting to flood and getting too much fuel so turn the choke back off again.

      Fuel flow issues can be caused by a faulty check valve (in the tank cap or tank body). If the valve is in the cap, loosen it a little and see if the engine will stay running.

    • profile image

      Ian 

      22 months ago

      Hi , my trimmer runs for a short time then cuts out. Restart and it runs for about 30sec and cuts out. Why?

      Please help.

      Ian

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      23 months ago from Ireland

      Hi Rajen,

      If the plug is burning, the engine must be running too hot. A mixture that is "lean", i.e. with too low a percentage of fuel in the fuel/air mix could cause this. On a lawn mower engine sometimes this causes the engine to spit and backfire. If there's a high speed mixture screw, you could open it up a bit, maybe 1/8 of a turn (noting how much you adjust to get back to square one in the future) and see if it improves matters over time.

    • profile image

      Rajen Govender 

      23 months ago

      Hi Eugene, thanks for the info. I have a 3 year old Riyobi brushcutter and this seems to be burning the spark plug. I have replaced 3 plugs within 2 months, cleaned the air filter and carb, but have not replaced kit in the carb. Would appreciate any advice

    • profile image

      Terry Howard 

      23 months ago

      thanks Eugene for the suggestions. With the old carby the symptoms were the same. The new carby looks exactly the same except for the mixture adjustment screw. As to the head for gravity feed I had the tank about metre higher than the engine. Will try the old carby again and replace the fuel line in case it is cracked or getting pinched as it passes through the grommet in the tank.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      23 months ago from Ireland

      Mysterious and mysteriouser! I wonder does gravity feed work with these types of carburetors? The pressure head you produce by rising the tank above the trimmer would would have to be sufficient to overcome the force of the metering spring pushing the needle into the seat.

      Do you think it would be worth replacing the old carby and doing all the tests again? Just in case the new one is a dud or it isn't totally compatible.

      Were the symptoms the same with the old carby?

      Are you absolutely sure there are no cracks in the intake fuel line in the tank? If you can, remove it and check it out.

    • profile image

      Terry Howard 

      23 months ago

      H I Eugene and thanks for the comments. Yes there is a mixture control on the new carby but not on the old one. I have tried adjusting the mixture from fully closed and then progressively by one full turn each time but no change to outcome. I cant understand why the engine runs on full choke, they usually only give a kick and you go to half choke for cold start and then fully open once engine warmed up. So I cant get by the thought that there must be an air leak that gives a burnable mixture with the choke closed but causes the mixture to go too lean once the choke or throttle is opened. I will try connecting a vacuum cleaner to the carby input side without an air filter to see if a partial vacuum is produced. At this time I cant think of anything else to do, very frustrating.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      23 months ago from Ireland

      Are there any mixture controls on the carby Terry and have you tried adjusting them?

    • profile image

      Terry Howard 

      23 months ago

      In your article you show how a diaphragm within the carby responds to fluctuations of pressure/vacuum and acts as a pump for the fuel supply to the carby. On my carby there is a small hole below the main throat and this aligns with a hole in the inlet manifold which goes into the crankcase. It is this hole that I believe is used to activate the pump in the carby. I have checked that there is no blockage and as I said I even tried a gravity feed to try and circumvent any problem with the pump despite it being a new carby but no change to the outcome.

    • eugbug profile imageAUTHOR

      Eugene Brennan 

      23 months ago from Ireland

      How does fuel enter the crankcase? If there's a valve maybe it could be obstructed?

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