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

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 a link to it on social media, Facebook, Pinterest etc.

Typical gas string trimmer

Typical gas string trimmer

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

  • Stale fuel
  • A dirty or cracked spark plug
  • Faulty magneto coil
  • 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
  • Bad compression. Checking this requires specialised pressure testing equipment.

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.

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.

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.

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

Oil mix ratios. For a 25:1 ratio, double the amount of oil

Amount of Fuel Mixture RequiredGasoline/PetrolTwo 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

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 or cause air bubbles in the line as air enters through the cracks. This can result in the engine stalling or make starting more difficult. Splits/cracks in lines mightn't be obvious. When I put my trimmer away after doing some cutting, even though it was stored with the tank on its back and fuel lower than the sealing bung where lines entered the tank, 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. This wasn't immediately obvious, so when checking lines for cracks, pull them out of the seal to make sure there aren't any in this section.

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

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

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 can be difficult to see a 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.

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.

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.

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.

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.

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

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.

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).

If you've ever emptied a 55 gallon oil drum, you'll know they have a secondary bung which needs to be loosened to vent the drum, allowing air to flow in and 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 is used (also called a check or non-return valve).

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 or the internal surfaces of the passageway in the valves could be stuck together, 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.

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.

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.

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.

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

Foam air filter.

Foam air filter.

Ensure the filter is free of dirt. Wash if necessary.

Ensure the filter is free of dirt. Wash if necessary.

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.

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.

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.

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.

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.

Remove the end of the throttle cable.

Remove the end of the throttle cable.

Carburetor ready for dismantling.

Carburetor ready for dismantling.

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.

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 filling the fuel lines and 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. Alternatively the the internal surfaces of the valve could be gummed together, preventing fuel from flowing through it.
  • The outer section is known as an umbrella valve and the floppy outer edges prevent fuel flowing backwards. When you release the bulb after pressing, 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.

To check whether the priming system is working, fill the tank so the line with the filter is submerged, but the other return line is above the fuel level. Then try priming. If fuel flows out the return line, the primer bulb and valves are likely working ok.

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.

One-way or check valve. Check there's no dirt stuck in the "duck bill" valve section in the center of the "umbrella" or the internal surfaces of the passageway through it aren't stuck together.

One-way or check valve. Check there's no dirt stuck in the "duck bill" valve section in the center of the "umbrella" or the internal surfaces of the passageway through it aren't stuck together.

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.

Fuel Pump

Fuel is pumped by pulses of suction from the crankcase acting through an "impulse hole" adjacent to the intake manifold. 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. Also when you replace any gaskets when re-fitting the carburetor, make sure they are the proper ones and don't block the impulse hole.

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.

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 as a result of exposure to ethanol in fuel, 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.

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.

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.

Remove the retaining screw.

Remove the retaining screw.

The needle valve seat. Clean this with carburetor cleaner.

The needle valve seat. Clean this with carburetor cleaner.

Needle, metering lever, and spring.

Needle, metering lever, and spring.

Clean All Surfaces Including the Jet and Screen Filter

Clean all surfaces and fuel passageways with a carburetor cleaning aerosol. This removes any gum deposits which can clog pathways and jets. The screen filter can be difficult to remove without prising up from the edge. This can cause damage, so it's probably best to leave it and spray cleaner down through it.

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.

Clean all surfaces of the carburetor with carb cleaner.

Clean all surfaces of the carburetor with 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.

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.

Cleaning a Two-Cycle Carburetor

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.

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.

Remove the cover over the exhaust outlet.

Remove the cover over the exhaust outlet.

Spark arrestor screen.

Spark arrestor screen.

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.

High Speed Mixture Adjustment

The high and low speed mixture screws are often indicated by the letters "H" and "L" respectively. These screws adjust the proportion of fuel in the fuel air mix. Turning them clockwise reduces the amount of fuel in the mix and vice versa.
Some trimmers don't have any mixture screws, the proportion being set in the factory.

To adjust the "H" screw, first screw it in carefully until it reaches the end of travel.

Screw out one to one and a half turns.

Try revving and if the engine runs ok, screw out by 1/8 turns until the engine starts to run rough when revving. Then screw back in a little at a time until it runs ok again.

The "L" screw can be adjusted in a similar fashion.

The procedure differs slightly between brands of trimmers, so ideally consult a service manual.

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 counter-clockwise about one eight of a turn and wait to see if the engine continues to run without cutting out.
  3. If it cuts out, turn the screw clockwise about one eight of a turn clockwise to increase speed
  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 counter-clockwise until movement stops.
Adjusting the idle speed.

Adjusting the idle speed.

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.

Crankcase Seals Leaking

This is another problem that can cause difficult starting. A leaking seal will reduce the engine's ability to suck fuel/air from the carburetor and cause it to suck air instead.

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.

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

Question: 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?

Answer: 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.

Question: Does the fuel tank of a 2-stroke engine tilt?

Answer: No. It's fixed to the engine. However, the pickup fuel line moves around in the tank so that it's always under the surface of the fuel, even if the tank isn't full.

Question: My toro weed wacker will only run with full choke. I just put a new carb on. Why is this?

Answer: It sounds as if it's starved of fuel and if it were an old carb, I'd suggest the following:

You might have:

Clogged jet/passages.

Clogged needle valve/seat

Check that any gaskets between the carb and air filter and carb and intake manifold are ok. There are one or two small holes beside the intake manifold on two-stroke engines, connected to the crankcase. Suction from the crankcase is used to operate the fuel pump in the carb. Check the hole/holes aren't blocked by the new gasket if it's a non-genuine part.

Check the fuel line in the tank to make sure there are no splits that could be letting air in and preventing its ability to suck up fuel.

Question: What could be the problem with my weed eater as the trim line trimmer won’t start and it’s very hard to pull the pull string?

Answer: Something could be causing the flywheel to jam, possibly a loose flywheel magneto. Remove the plug and see if it's easier to pull the starter cord. If it is, check the exhaust and air cleaner to make sure they're not clogged and restricting the flow of exhaust out and mixture flowing in. There may just be high compression on the engine (some engines have a compression release that reduces compression and makes it easier to pull start the engine).

Question: Why is my priming bulb hard to press?

Answer: If the valve is like the red one in the photo in the article, it could be because it's stuck, specifically the central "duck bill" part that allows fuel to pass into the reservoir pocket under the diaphragm. If it stays stuck in, it's the flappy edges that are stuck.

Question: How can I fix the magneto on my string trimmer?

Answer: There's very little you can do with the magneto other than making sure the distance from the laminated core to the flywheel is correct and tightening the mounting screws. Loose connections should be checked and screws tightened on ring crimps. Faulty magnetos that don't produce a spark at the plug (always try a plug that you know is working and isn't cracked) should be replaced with a new unit.

Question: Do you have any possible causes for a dangerously strong recoil on the pull cord? The engine engages, whips the cord back and then the engine cuts.

Answer: This can be caused by the engine misfiring at the wrong point in the sequence. So instead of the plug sparking and the mixture exploding as the mixture is compressed and the piston is about to change direction, ignition occurs prematurely, and the piston gets pushed back "the other way." So this turns the engine the wrong way and pulls the cord back.

Misfiring can be caused by a loose or incorrectly adjusted magnet coil, or a flywheel in the wrong position on the crankshaft due to a damaged flywheel key. Also, poor fuel and air leaks between carburetor and engine can be an issue.

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

Answer: 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.

Question: Can you provide a diagram of the diaphragm, needle valve combination in a carburetor?

Answer: I don't have a diagram, but if you look below the section in the article "Diaphragm on a Two-Stroke Engine Carburetor", you'll see photos of the arrangement.

Question: Can the exhaust valve not open on a Mantis tiller?

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

Question: Why does the starter cord on my string trimmer snap back out of my hand when starting?

Answer: This is likely due to misfiring and the engine turning the "wrong way". So it can fire once and pull the rope back.

Misfiring can be caused by a loose or incorrectly adjusted magnet coil, or a flywheel in the wrong position on the crankshaft due to a damaged flywheel key.

Question: Can you to explain why burned fuel exits by the admission of new mixture into the cylinder?

Answer: When the piston descends into the crankcase, it pushes mixture previously admitted into the crankcase up along a passageway into the cylinder. This flushes burned gasses out of the cylinder.

Question: What is the carburetor model shown in your article and can you get a refurbishing kit for it? It's the same as mine.

Answer: I had a conversation with a reader called Mike about 15 months ago, and he reckoned the carburetor was a Walbro wyl series carb. Search for "Mike" on the page and you should be able to find it. He got a rebuild kit for the trimmer and it worked ok.

Question: When I pull the starter rope I get a clanking noise.The engine quit like it was fuel starved. Why does this happen?

Answer: It sounds like something's rubbing or broken in the engine. It could be a serious failure like a broken connecting rod (which you can check by removing the crankcase cover) or possibly a loose magneto coil rubbing against the flywheel, requiring the fixings to be tightened, ensuring the legs of the coil's former are at the correct spacing distance from the flywheel magnets.

Question: Why is it difficult to pull the starter cord but easy with the plug removed?

Answer: A clogged exhaust can make it difficult to pull the starter cord. Over time, residue from combustion can clog exhaust ports, spark arrestor screen and muffler.

Question: What causes difficult pull starting?

Answer: The magneto/coil may be rubbing the flywheel. Also, there could be a hydraulic lock with fuel in the cylinder. If the engine doesn't have a clutch, whatever is down the line from the engine may be sticking and making pull starting difficult. Thee could also be an issue with the recoil starter, causing the rope, spring mechanism or starter pulley to jam.

Question: Why does my MTD grass trimmer keep stalling?

Answer: There could be several reasons:

- A dirty fuel filter

- Water in fuel

- Cracked intake fuel line

- An ignition coil that is starting to fail

- A bad diaphragm

The line that sucks up fuel in the tank is supposed to move around when the trimmer is used at different orientations. If it doesn't do this, if the fuel is low in the tank when the trimmer is used at an angle, or upside down, air can get sucked into the line and stall the engine.

On initial starting, a trimmer can splutter and stall with choke off until it warms up and reaches speed and the carburetor fills properly. Try starting and running with choke back on again or half on for several seconds before turning the choke back off. See the guide for more details.

Question: How do I check the oil level in an engine?

Answer: 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.

Question: I have started using my Mitox 2-stroke petrol strimmer to cut down very tall, dense grass. So I've fitted the heaviest duty nylon line, and the strimmer is working very hard. Yesterday was very humid, and I noticed water coming from the exhaust in quite large amounts. I initially thought the petrol tank has split, but was reassured to find that the fluid was dirty water. Is this normal?

Answer: A by-product of combustion is water vapor, 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. It's possible as the exhaust gasses expand, the temperature drops to the degree that water vapor condenses out in the humid atmosphere.

Temperature can drop substantially as gas expands, hence the vapor trails on the wings of aircraft in low-pressure regions of the wing, or you'll notice how an aerosol can get really cold as the spray is released. Another example is when you blow out air from your mouth onto your 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 of the temperature drops as it expands on exit. Moisture will also condense when the 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.

Question: Why does my trimmer leak from where the lines come out of the tank?

Answer: Lines get stiff and shrink over time from exposure to gas. Therefore to stop this type of leak, it's a good idea to replace both lines. If you're doing this repair yourself, drain the tank first, then push the lines off the carburetor connections using the side of the blade of a screwdriver. Next, pull them through the grommet and remove them from the tank. Remove the grommet also, ideally using your thumb to peel it out by the edges rather than using a screwdriver which could mark the sealing surfaces, potentially resulting in a leak when it's replaced. Bring the trimmer or grommet to your local hire/repair shop and get them to provide you with a fuel line that's a tight fit for the grommet. When replacing the lines, feed them through the hole in the tank first with the grommet removed. Then push both lines through the grommet, leaving about 1/2 inch feeding through. It's easier to pull lines than push because they stretch and reduce slightly in diameter, whereas they bulge when being pushed. Next replace the grommet in the tank. If you find this difficult, use a screwdriver to poke it in at the edges, but use the side of the screwdriver blade rather than the tip, again to avoid damage. Pull out the lines and connect them to the carburetor inputs, making sure they're the correct way around.

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

Answer: 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:

© 2014 Eugene Brennan

Do you have any questions or comments to add?

Sylvain Gagnon on August 20, 2020:

Hi, thanks for the quick reply and taking the time to look into it. Mine doesn't have any marking either but I was hoping your manual had a part diagram with part numbers. Mine didn't. I'll call Yardworks but I'm afraid I'll be told that it's discontinued and can't help me. I'll keep Googling and maybe I'll get lucky.

BTW, reason I need a new carb is after cleaning it in my ultrasonic tank and rinsing it, I used compressed air to get the water out. Don't know if you know this but there is a small o-ring within that cross air hole. The compress air took it out and there is no way to put it back. That part doesn't come off either so my carb is having more issues than it did before messing with it :-(

Eugene Brennan (author) from Ireland on August 19, 2020:

The only thing I can suggest is to try and blow out any flakes that could be lodged in passage ways in the carb with an air compressor blow gun. Blow out the jet in the carb too. Check the valve in the primer button is clear too and pushing air out through the centre and not sucking it in from the tank through the flappy edges I describe in the article. Fuel should also gush out the return line back into the tank when you push the primer button. on August 19, 2020:

Great article, tons of good information. Didn't answer the question about if it is normal for the primer bulb to have a large air bubble no matter how many times you press it in. My Blue Max was new, purchased prior to my shoulder surgery so it wasn't unboxed until a year later. It started once. It would idle but never gut a single weed as would conked out when the trigger was depressed. Due to 2nd shoulder surgery I drained the gas out and put it away. Now this August I did all the basic including taking apart and cleaning the carb and blowing out the gas filter. My findings were only that there were black gaskets flakes like from a dirty factory that made the carburetors. The carb screen was clean but flakes beyond in other areas. Put gas in and it almost started then nothing after many pulls. I have a tack for my chainsaw and lawnmower repairs. A pull yields an RPM of 600. Last week it would run if you feed it gas with an eye dropper directly into the carburetor. I figured cleaning the carb would solve the problem even though there are zero hours on the machine. Not starting even with 1/16 CCW turns on the 1 carb adjustment screw. I bought the Blue Max because it had good reviews and at 42 CC's had more power than the models I saw at Home Depot. Hope you can help. Only recourse seems to be to buy another carburetor for $14 as I'm quite sure it is not timing since it runs if you feed it gas with the eye dropper.

Eugene Brennan (author) from Ireland on August 19, 2020:

Hi Sylvain,

Unfortunately there are no markings on my carb. I can't find any references online for your carb either, but I've tweeted Canadian Tire and they may be able to help.

Update: They advised ringing the Yardworks customer service phone line at 1-866-523-5218

Sylvain Gagnon on August 18, 2020:

Hi, thanks for the great article. It really explains how this little carb is working and that video you linked was very informative as well.

One question, I have a Yardworks 060-2326-0 25 cc trimmer with engine WL1E33F. I can't find a match for a carb anywhere online. It does, however, looks EXACTLY like the one pictured here. Would you have its part number or better yet, would you know the part number of the carb for this Yardworks trimmer?


Eugene Brennan (author) from Ireland on July 02, 2020:

The exhaust could be clogged or the recoil starter could be rubbing creating friction which would add to the normal force required for starting. Some strimmers have decompression valves which need to be pressed also when starting. Have a look at this discussion:

Paul Carty on July 02, 2020:

My strimmer cut out whilst using it. I tried restarting it but the pull chord was very stiff. When the spark plug is removed everything moves smoothly. Any ideas?

Ravish Gandecha on June 29, 2020:

Hello Eugene,

Interesting article. Since most of the string trimmers and other such handheld devices are having 2-stroke engines predominantly. Can you please tell me roughly what can be split (out of 100%) in the market between 2 and 4 stroke in string trimmers, or other handheld equipment such as chainsaws. Is there a market shift from towards 4 stroke from 2 stroke in any handheld equipment you see?



Eugene Brennan (author) from Ireland on June 28, 2020:

Hi Richard,

Thanks for that.

This may have resolved the problem temporarily allowing air into the tank, but there could still be an issue with the check valve (located in the cap or body of the tank). Probably a good idea to examine it to see if the internal valve surfaces are stuck together or there's stuff stuck in it.

Richard Fryer on June 28, 2020:

Hi Eugene. Thanks for the advice. I unscrewed the fuel cap and that has sorted the problem. I thought it should be something simple as I always use the correct fuel mix and being almost new very unlikely to be a problem with the carb. Great article.

Eugene Brennan (author) from Ireland on June 27, 2020:

This model seems to have a clutch. Is it possible that the cutting head or shaft are seized and this stalls the engine when it's revved?

Are you using new fuel?

Check the exhaust screen isn't clogged with carbon.

The crankcase seal could be compromised causing the engine to suck air.

How did you adjust the high speed mixture screw?

Screw it in carefully until it reaches the end of travel.

Screw out one to one and a half turns.

Try revving and if the engine runs ok, screw out by 1/8 turns until the engine starts to run rough when revving. Then screw back in a little at a time until it runs ok again.

Do the same for the idle screw.

Aaargh128LD on June 27, 2020:

Hi Eugene, thanks for your comments. Yes we've adjusted the high speed mixture - but no joy there (ok we can get it going better, but it still cuts out). We've also tried the fuel cap trick - again no difference. We've also tried the gentle squeeze trick - again no joy. Interested to read your comments on the coil - so maybe this could e the culprit? But yes the issue is just with throttling - it idles fine. It's an odd one as we have changed everything else and cleaned meticulously.

Richard Fryer on June 26, 2020:

I have a Stihl Kombi engine which is only a couple of months old. when tilted forward it starts to cut out. It starts ok. Wondered what is the most likely problem.

Eugene Brennan (author) from Ireland on June 26, 2020:

Maybe the coil just ceases to function due to electrical break down when the engine speeds up and it's sparking more often. I don't know. If the engine cuts out when throttled, it's due to fuel starvation for whatever reason. Is there a high speed mixture screw that you can adjust? Have you tried loosening the fuel tank cap in case the tank isn't venting and letting air in as fuel leaves to feed the carburetor? Sometimes my strimmer dies on throttling up, so after turning off the choke, I gradually squeeze the throttle and let it reach speed (and turn the choke back on half way if it starts spluttering).

Aaargh128LD on June 26, 2020:

Great article btw - So have a Husq 128ld strimmer which dies after throttling. Have replaced carb, fuel filter, fuel lines, spark, filter and cleaned it top to bottom. I can now start it and it will idle, but as soon as I try to throttle it dies. I have repeatedly adjusted the fine tune screws which don't seem to help (occasionally it may run for 60 secs, but then dies). Fuel is new - everything new.

Have taken it to local 'fixer' who suggested that ignition coil module 'may be' required.

My question is that as it starts, idles etc - would the coil actually have any bearing on the running of the engine?

Happy to hear if so, but don't want to pay for someething to be replaced if there is no way it could be the problem?

Your advice would be welcomed.

Eugene Brennan (author) from Ireland on June 22, 2020:

Hi Matt,

Sorry I don't. The only way would be to try and get the information on the supplier/manufacturer from the retailer you bought it from, but usually they're reluctant to give that out.

Matt Reynolds on June 21, 2020:

Just wondering if you had a motor parts list. Mine is identical to yours but a different brand (still made in China) and of course I can't find anything on mine. Really excellent article you done. Really appreciate it.

Eugene Brennan (author) from Ireland on June 20, 2020:

Hi Harry,

I'm out of suggestions too.

I have an issue with my trimmer at the moment. I think what happens is that the carburettor leaks in storage. I'm not sure of what exactly happens because I store it with the tank on it's back, so possibly as temperature rises in the shed, this causes fuel to expand in the lines and dribble out past the metering needle. Anyway with fuel flooded into the carburetor and crankcase, it's difficult to start. It eventually starts by leaving the choke off and then alternating between choke off and choke on and there's lots of smoke as excess fuel is burned off. I find when an engine is struggling, it helps to alternate the position of the choke as soon as it runs. So adjust on or off and quickly change positions if the engine starts to die. Also don't rev an engine until it's running for a while as that often quickly cuts it out.

Have you tried adjusting the mixture screws? If you do, turn the screw first until it reaches the end of travel (usually clockwise) and count the number of turns so you can return it to the original position if there's no improvement (otherwise you'll have an additional problem which will be even more difficult to resolve). I'm not sure what the adjustment procedure is for mixture adjustment. I've linked to a Stihl procedure in the article, but it may be different depending on brand.

wesley on June 20, 2020:

i have a ryobi rpt 3000 string trimmer and it sucks up to much fuel any advice

Harry on June 19, 2020:

Hi Eugene, Thanks for the info. Here is the latest.

Checked the spark plug - it is generating a spark (tested by unscrewing it but keeping it plugged into the rubber section and grounding - you can see spark).

Checked the gasket - in place and cut-out for opening to crankcase.

Checked the fuel needle - looked OK to me, no deposits.

Checked the spark arrestor - screen looked good; no blockage or debris.

Finally, I tried spraying some starter fluid directly into the carburetor and interesting it started for a few seconds then stopped. I tried few more times and same result. Also, it seems when pulling on the starter cord, it seems like its about to start but then I hear a clicking sound and it locks.

Any other suggestions (I'm at my wits end with this and wondering if its worth more time)

Thanks again!

Eddie416 on June 18, 2020:

Howdy.........Well the trimmer has been used many times since the original problem. I'm sure is was a basic lube problem. I did have the right fuel/oil mix when I used it 7 months ago, but I didn't run the tank dry or put a spoon full of oil into the combustion chamber at storage time. Lesson learned here for sure. I thank Eugene for this very fine and helpful site. !!! :-))))

Eugene Brennan (author) from Ireland on June 18, 2020:

Hi Harry,

Sounds like you've covered most things. Is the trimmer generating a spark ok? Is the carburetor well secured to the engine with the gasket in place? If the carburetor kit came with a gasket for fitting between it and the intake manifold on the engine, is it exactly the same? The intake has a small hole adjacent to it normally which is the opening to a passageway to the crankcase. Suction from the crankcase operates the fuel pump in the carburetor. Did you check the fuel needle in the carburetor? Was it sticking or was the tip coated with any deposits that could have stopped it sealing?

Harry on June 17, 2020:

Hi Eugene! - great article, very informative and helpful. Wondering if you could help me. I bought a Husqvarna 128CD grass trimmer a few years ago. The first year was great, but unfortunately my novice mistake was to leave the gas in the tank over winter the first year. The second year it stopped working and I put it aside. I'm trying to get it working again. I've have bought a carburetor replacement kit and changed the carb, fuel lines, fuel filter, air filter, primer bulb, added new gas with correct oil ratio. Still no luck starting the machine. I've followed guideline to prime bulb, pulled the starter cord 10x, put in choke mode - no luck. Any suggestions would be helpful. Thanks.

Eugene Brennan (author) from Ireland on June 12, 2020:

Thanks James. Yes, that's true they do perish, both rubber and synthetic. I have used air line as fuel line on my hedge trimmer and haven't had to replace it in ten years. Not sure what the material is. I tried polyurethane line and it last well but does eventually deteriorate. I think this stuff may be nylon. The only problem is it's a bit rigid, so the line with the filter doesn't drop to the bottom of the tank very well when the trimmer is used at different angles. However I just keep the tank at least half full.

James Burke 2020 on June 12, 2020:

My advice is to also check the plastic fuel lines. They perish and leak, even if not used very much. This happened to both my 2 stroke chainsaw and 2 stroke strimmer. When you press the primer bulb, look carefully for leaks or bubbles on the tubes. Mine were just about long enough to cut down and re-seat but you can buy the tubing in motor spares shops (bring a bit with you). Great article by the way Eugene.

Eugene Brennan (author) from Ireland on May 25, 2020:

That's odd. I've never experienced that with my strimmer. Was there any corrosion visible on the piston/cylinder that could have caused sticking? Is your climate very humid which would promote corrosion? It's recommended that a dessert spoon of oil is poured into the spark plug hole of a lawn mower engine before winter storage to coat surfaces and prevent corrosion. Are you definitely adding the correct amount of oil to the fuel mix? If you've used old petrol/gas mix, did you swirl it before use because oil can separate out and settle over time.

Eddie416 on May 25, 2020:

OK, here is an update on this problem. I dissembled down to just leaving the cylinder head on. Using a bright light I could not see much carbon in the chamber area. I could see the piston through the Carb and exhaust ports and I put a few drops of oil through each port to give it some lube and rotated the crank a few times. I couldn't see anything wrong so I reassembled the engine. This time it was much easier to pull the starting cord and after a few pulls it started. I think lubing the piston was the key. This engine sat idle for the last 6 months and even though it seemed to turn over easy, it needed more oil to be able to "spin" through several crank revs on just one cord pull.

Eddie416 on May 15, 2020:

Thanks for your answer Eugene. As I mentioned before the Carb and Muffler have been removed. If I turn the engine over by hand I can tell it is just as hard to get past TDC as was when the 2 items were still mounted. I wonder if there could be a huge amount of carbon the the combustion chamber to cause the compression to go so high. ????

Eugene Brennan (author) from Ireland on May 15, 2020:

Check the air filter to make sure there's no restriction. Check the exhaust also just in case there's any carbon deposits that's clogging it.

Eddie416 on May 15, 2020:

I have a 2007 Stihl FS 55 R string trimmer that is impossible to start. In fact I can't even pull the cord to make the crank "spin" over enough to energize the coil. If I step in the handle to hold it down I can get only one 360 degree turn of the crank, and it feels like I am going to break or pull the cord right out of the machine. I have removed the muffler and carb and it is still just as hard to turn the crank to make the piston go over TDC on compression and for intake going back down. With the plug removed it spins easily with no friction at all. I can see the piston traveling from both sides of the cylinder and it all looks smooth. Bottom line is the compression seems to be to high, but I don’t see how that could change at all. Start rope, recoil spring, and housing all are fine. Any help would be appreciated.

Eugene Brennan (author) from Ireland on May 09, 2020:

I Googled "cap for Chinese strimmer" and several products turned up on Amazon, eBay and other sites. One product which was advertised on a lot of sites was described as "Fuel Tank Cap For 33CC 43CC 52CC CG330 CG430 CG520 BC430 BC520". It had an internal diameter of 36mm.

Eugene Brennan (author) from Ireland on May 08, 2020:

Hi Rob,

Have you any idea where the machine was bought? If it was a homestore own brand model, probably manufactured in China, there's a slim chance the store may be able to get a spare part. This is the problem with getting an unknown brand, parts are difficult to get. Maybe if you measure the external diameter of the threads on the tank and also the pitch (measure the distance between as many threads as possible, then work out how many are in an inch or the spacing between threads), it might help if they give these specs online. I tried to get parts for this machine, but the store I bought it in wouldn't tell me who the manufacturers are in China.

robflan on May 08, 2020:

Hi thank you for this great article. I have just bought the exact same strimmer you have. I got it second hand and when i got home i just realized the fuel cap isn't correct. I bought a fuel cap online but it doesn't fit. Can you advise where i might get one or is there a model I need to look out for? Any help will be much appreciated.

Eugene Brennan (author) from Ireland on May 02, 2020:

Hi Graham,

If the fuel tank cap vent isn't at fault, possibly the coil is breaking down when the engine gets hot? You could try to see if you're getting a spark (make sure the gas cap is shut and there's nothing flammable around). It's best to test in the shade and you may need an assistant to see the plug while you're pulling. The plug has to be grounded also to the metal body of the engine which can be awkward to do.

Graham Peck on May 02, 2020:


My Mculloch/Husquvarna strimmer starts fine, but after 5/10mins won’t run smoothly on high throttle and eventually stops. Had new fuel lines last year, mostly ok til now. Some Helpstart in the 2 stroke mix, used in my 4stroke mower. Have loosened fuel cap to to if any starvation but same problem. Given the carb a clean but the diaphragm gasket looks too fragile to lift...any suggestions please?

Mark Harrison on April 20, 2020:

Hi Eugene,

Thanks for the link. I didn't find that one while doing my own searching yesterday. Turns out that your seller is in Oregon and I am in So. California, so it could work.

Here's a picture of the head end of the aluminum drive shaft for my trimmer: There is no way to stick anything up from this end without drilling a hole first. I was asking about removing the end mechanism, but realize I would probably warp the drive shaft and/or destroy the bearing that the head rotates on in the process.

So, I took a look at the other end and was able to grab hold of the nylon/plastic tube you can see sticking out of the drive shaft in this image: with some needle nose pliers and proceeded to pull it out. It's basically a tube that the flexible shaft is housed in with 4 fins that keep the tube centered in the drive shaft. Wasn't sure how easily it would come out or if I could get it back in, but some WD40 helped with that. So, its nearly as long as the drive shaft and once I had removed it, I was able to tap at the head end and shake the broken end piece out of the drive shaft.

Here's a final picture that shows both pieces of the flexible shaft at the broken end: Both ends of the shaft are well square so I am confident that I can now replace it and get the trimmer running again. Was able to make some calls today and think I have a local source for the new part. Thanks again for your guidance.

Eugene Brennan (author) from Ireland on April 19, 2020:

Hi Mark,

For what it's worth, here's a link to a shaft for sale I found on eBay. Replace the word "dot" in the URL below by ".". We have to keep links to sites to a minimum here to keep Google happy as regard page ranking in SERPS.

Has a section of the shaft actually broken? Have you removed a piece of it? Is the remaining piece jammed? Can you get a vise-grips or long nose pliers on the end of it to twist and pull it out?

https://www dot

Edit: Just noticed that product is being sold on eBay UK, so carriage would be prohibitive if you're in the US.

Edit Edit: I understand now what you mean. A piece is left in the shaft. Can you poke it out with a piece of thick wire? I'll try to think of something else that would be flexible. (maybe a length of 1/2" or smaller diameter PEX?)

Mark Harrison on April 19, 2020:

Hi Eugene,

Your guidance back in June of 2017 helped me get my Homelite Trim N' Edge String Trimmer, Model UT-20702, working again and I thank you heartily for that.

My new problem is that the flexible shaft, part no. UP03031, has broken. The frayed end is at the head and the long piece is ~48.5 inches, which leaves ~1/2" still in the curved shaft.

I have two questions for you:

1) How do I get the broken end out of the shaft, or is that not possible?

2) Since my online searching has revealed that the original part number and both subsequent substitute numbers (PS01502 or 986117001) that I've been able to find have been discontinued, is it time to retire this trimmer?

Thanks in advance for your guidance once again.

Eugene Brennan (author) from Ireland on April 18, 2020:

Hi Francine. No it usually can't because the throttle linkage probably won't align. Also there's a duct to the crankcase on the manifold that produces the pulses of suction that pump fuel through the carburetor. The entry to this may not mate with the corresponding hole on the carburetor.

Francine on April 17, 2020:

Can a carbie be installed upside down on line trimmmee?

Eugene Brennan (author) from Ireland on April 11, 2020:

Hi Stuart, can you repost and delete your email address? If the bulb doesn't fill up, it could be a problem with the valve that lets fuel into the bulb. Check the edges aren't stuck. Could it be that the bulb isn't the correct part and the rim at the base is too thick, stopping any valve from opening? (if the valve is like the umbrella one fitted to my trimmer).

Eugene Brennan (author) from Ireland on March 12, 2020:

Hi Angie,

It's best to empty out the old fuel (and dispose of safely). If the strimmer has a primer button, press it several times after you empty out the old fuel to make sure any fuel stored in the carburetor is gone. (You'll see it squirting out the return line in the tank, the one without a filter on the end). Then use new fuel. The machine should be safe to use (However check before hand that there are no leaks. Fuel lines can become brittle, deteriorate and crack over time and a leak is a fire hazard)

Angie on March 12, 2020:

Hi, Eugene , I have brand new strimmer which had only been used once and had then been left for two years lying flat in the shed with mixed petrol in its tank. What should I do? Would it be safe to use? :( My husband wants to know.. and he would to let you know too that he had enjoyed your directions .:)

Eugene Brennan (author) from Ireland on February 23, 2020:

Hi Vicky, don't have any info on timing. Is your choke definitely turned off when it's running? This discussion suggests that it can be due to a worn piston skirt.

Vicky on February 23, 2020:

Hi i have a bush cutter L-155 chinese make it starts like a dream nut spits feul out the carb can you help me figure it out and do you have any tips on timing for piston its a 2 stroke motor

Eugene Brennan (author) from Ireland on January 30, 2020:

Hi Matt,

The only other things I can suggest are to check the primer bulb to make sure it is sucking up fuel and returning it to the tank (look into the tank and check fuel is coming out the return line). Make sure the carburetor is sealing tight against the engine. Loosen the gas tank cap slightly to make sure the check valve isn't an issue. Check there are no cracks in the fuel lines that could be letting in air (on my trimmer a line was cracked at the point where it passed through a grommet into the tank. The compression could be compromised on the engine, there should be quite a bit of resistance when you pull start it. The crankcase also has a gasket and this mightn't be sealing properly either. A repair shop would be able to do a compression test.

Matt on January 29, 2020:

I have a Troy-Bilt TB32 weedwacker that won't start. I got it used recently for my son, it worked great for about an hour's worth of time, then in the middle of running it just stopped. It turns out that the piston seized somehow. I eventually got it loose after spraying a lot of PB Blaster and working it loose.

After it got loose, we still couldn't get it to start. I checked everything I could think of. The gas & oil mix is relatively fresh, I can hear a compression now when pulling the cord, it has a new sparkplug with the right gap, I can see a spark when I connected a plug tester to the sparkplug boot, the carburetor looked clean, the air filter looked clean. I added starter fluid to the carburetor and also to the piston and it never tried to turn over.

I can't figure out what else to test. Could the fact that I used a lot of PB Blaster be messing it up? Anything else I should try?

Eugene Brennan (author) from Ireland on November 18, 2019:

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).

Wallace Tom on November 18, 2019:

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.

Eugene Brennan (author) from Ireland on November 18, 2019:

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).

wallace tom on November 18, 2019:

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

mikealpha on November 18, 2019:

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!

Eugene Brennan (author) from Ireland on November 17, 2019:

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.

Eugene Brennan (author) from Ireland on November 02, 2019:

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

Dale on November 02, 2019:

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,

Eugene Brennan (author) from Ireland on October 21, 2019:

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.

Dale on October 20, 2019:

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.

Craig on October 11, 2019:

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.

Eugene Brennan (author) from Ireland on October 10, 2019:

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

Don from Gelorup, Western Australia on October 10, 2019:

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.

Eugene Brennan (author) from Ireland on August 28, 2019:

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

Jason on August 28, 2019:

Weedearer crank wont stay running

Eugene Brennan (author) from Ireland on August 01, 2019:

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.:

paulWILIAMS on August 01, 2019:


Chris V Sexton on July 20, 2019:

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.

Eugene Brennan (author) from Ireland on July 08, 2019:

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 on July 08, 2019:

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

mikealpha on June 01, 2019:

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

Eugene Brennan (author) from Ireland on June 01, 2019:

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.

mikealpha on June 01, 2019:

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

Eugene Brennan (author) from Ireland on May 16, 2019:


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.

JohnnyJ on May 15, 2019:

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.

Dean1978 on May 12, 2019:

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

Eugene Brennan (author) from Ireland on May 12, 2019:

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.

Dean1978 on May 12, 2019:

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.

Eugene Brennan (author) from Ireland on May 04, 2019:

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.

Nigel Cross on May 04, 2019:

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

Eugene Brennan (author) from Ireland on May 02, 2019:

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)

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

Mick on May 02, 2019:

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.

Eugene Brennan (author) from Ireland on April 21, 2019:

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.

Milesa petrovic on April 21, 2019:

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

Eugene Brennan (author) from Ireland on February 19, 2019:

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.

Tom Tefau on February 18, 2019:

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

Charl on February 01, 2019:

Thanks very much very helpful

Eugene Brennan (author) from Ireland on January 18, 2019:

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.

Alen on January 18, 2019:

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?

Eugene Brennan (author) from Ireland on January 10, 2019:

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.

Jay on January 10, 2019:

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?

Eugene Brennan (author) from Ireland on December 31, 2018:

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.

G on December 30, 2018:

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


ryan on December 15, 2018:

my wheateater didnt wanna spin.

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

Can you help me?

Eugene Brennan (author) from Ireland on October 19, 2018:

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.

Gary abdul on October 18, 2018:

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

Eugene Brennan (author) from Ireland on September 28, 2018:

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.

Paul on September 28, 2018:

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

Eugene Brennan (author) from Ireland on August 26, 2018:

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.

Eugene Brennan (author) from Ireland on July 02, 2018:

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.

John Harrod on July 02, 2018:

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

Eugene Brennan (author) from Ireland on June 05, 2018:

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

Sheik on June 05, 2018:

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