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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.

Why Won't My String Trimmer 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 explore some of the issues which can affect these machines. I strip and clean a carburetor and explain how it works and point out where problems can arise.

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

Garden Tools That Use Two Stroke Engines

While I specifically deal with the carburetor from a trimmer, these tips may also help for troubleshooting other two-stroke engine-powered machines. This type of engine is typically used on:

  • brush cutters
  • hedge cutters/trimmers
  • chainsaws
  • consaws
  • leaf blowers

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.

A carburetor works on Bernoulli's Principle. As air moves faster, static pressure decreases causing fuel to be sucked into the carburetor and mix with the airflow coming from the air filter.

  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.
  5. A choke plate at the intake of the carburetor increases the ratio of fuel in the air/fuel mixture when it is closed, to aid starting. It's either closed manually by a separate control lever or when the trigger/throttle control is set to the start position.
  6. A throttle plate varies the amount of mixture reaching the engine to control power and speed. Some engines don't have a choke plate and vary the amount of spray leaving the jet. The angle of this plate is varied as a user squeezes the trigger control.
Carburetor. Image from Wikimedia Commons cropped, edited and annotations added.

Carburetor. Image from Wikimedia Commons cropped, edited and annotations added.

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.