String Trimmer (Strimmer) Won't Work: 2-Stroke Engine and Carburetor Troubleshooting
String Trimmer Won't Start?
String trimmers, also known as line trimmers, weed eaters, weed whackers, strimmers, and whipper snippers, can be troublesome garden tools. Difficulty in starting is often due to an issue with the carburetor (carburator).
In this article, I offer a list of troubleshooting questions to help you find the problem. I strip and clean a carburetor and point out where problems can arise.
While I specifically deal with the carburetor from a trimmer, these tips will also help for carburetors on other two-stroke engine-powered machines (such as brush cutters, hedge cutters/trimmers, chainsaws, consaws, and leaf blowers).
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Common Reasons Why a String Trimmer Won't Start
- A dirty or cracked spark plug
- The magneto isn't generating a spark
- Improper venting of the cap on the gas tank
- A blocked fuel filter
- A very dirty air filter
- Clogging of the carburetor from gum deposits due to gas being left in the tank over winter
- A worn diaphragm or pump in the carburetor
- A stuck valve in the primer bulb or a leaking bulb
- A blocked spark arrestor screen
Usually though, the problem is caused by parts which wear out or become compromised in some way in the carburetor. Before jumping to conclusions and starting to troubleshoot, follow a proper start sequence.
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.
- 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.
- 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.
- 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.
- 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
How to Fix a String Trimmer
I have a bargain basement trimmer I bought in a large home store. Unfortunately, spare parts are unlikely to be widely available for repairing such a model (although the store may be able to get them from their supplier), which was probably mass produced in China. Still, it was cheap and I've got years of use out of it with no engine problems (except the protection guard for the line broke and I had to make a new one).
If you want a machine to last for 10 years or more, go for a model by a well-known manufacturer such as McCulloch, Stihl, Husqvarna, Poulan Pro, Oleo-Mac, Ryobi, Craftsman, Weedeater, Flymo, or Honda. That way, you can be certain of getting service information, spare parts, and after-sale support.
Warning: Gas Is Flammable
Gas (petrol) is flammable. Keep all sources of ignition away from where you are working on an engine. This includes naked flames, cigarettes, hot surfaces, and tools which generate sparks (e.g. angle grinders). Rags soaked in gas can ignite, so dispose of them safely. Store gas in metal cans or plastic containers made for the purpose.
How to Mix Fuel for Two-Stroke Engines
Don't use four-stroke motor oil in your engine. It contains additives which can contaminate the cylinder over time and also it can tar up the piston. Two-stroke oil is designed to burn as cleanly as possible with the fuel.
- Lubrication of two-stroke engines is effected by mixing two-stroke oil with gas before use.
- In general, the oil-to-gas ratio for a two-stroke engine should be 1:50. This equates to 20 ml per liter (litre) or 2.6 fluid ounces (US) per gallon (US).
- Some manufacturers recommend a 1:25 ratio for their engines, so check your manual.
- 1:40 or 1: 30 is probably a good compromise (i.e. more oil).
- Try to avoid fuels which contain ethanol as this absorbs moisture over time, oxidizing metal surfaces.
- Swirl the mixture in the container for a few seconds to mix it thoroughly.
- You can normally buy 100 mL plastic bottles of two-stroke oil which is sufficient to make up 5 litres of mix. In the US, 2.6 fluid ounce bottles of oil are available for making up a gallon of mix. If you do a lot of trimming, obviously it's more economical to buy a larger bottle of oil.
Gas and Two-Stroke Oil Needed For a 50:1 Ratio
Amount of Fuel Mixture Required
Two Stroke Oil
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
1 US gallon = 3.79 litres
1 UK imperial gallon = 4.54 litres
1 UK gallon = 1.2 US gallons
1 UK pint = 20 UK fluid ounces
Starting a Trimmer, Hedge Cutter, or Chain Saw Properly
Priming the Carburetor
- Before you start your engine, you need to prime it. The priming bulb sucks fuel up into the carburetor which may be empty from the last time the engine was run.
- The priming bulb must be pushed about ten times to suck in sufficient fuel. Make sure there is enough fuel in the tank before you do this so that the filter on the end of the fuel line is submerged and you're not just sucking air into the priming bulb.
- The engine only needs to be primed again after a tank refill, if it has been a long time since the trimmer has been used and the carburetor has drained, or if the trimmer was used upside down for an extended period and the engine cut out.
- The filter on the fuel line is normally able to move around in the tank so that even when the trimmer is used upside down, the filter falls down into fuel.
- However, if there isn't much fuel in the tank, sometimes this doesn't work, so the intake line can get drained and filled with air as the engine uses up fuel, requiring a re-prime. Try to keep the tank at least half full.
Starting the Engine
- Rest the machine on the ground, clear of any debris, and start the engine on full throttle with the choke on.
- If it doesn't start after a few pulls, try re-priming.
- Once the engine gives a kick, give it another pull and if it runs, wait until it cuts out or for about ten seconds, whichever comes first.
- Then turn off the choke and allow the engine to continue running (or try starting again if it had cut out on-choke).
- If the engine won't start, repeat the process from the start by turning the choke back on, priming the engine, and pull-starting.
- 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.
- If the engine has been run in the last 10 minutes or so and is hot, you don't need to turn the choke back on before re-starting.
Note: There is some variability in the starting procedure for this type of engine. Some engines don't have a primer bulb. Also, an engine may have both a partial and full choke position. The choke may be arranged so that you start the engine without squeezing the throttle and revving it, only doing so when it fires up. Revving then automatically knocks off the choke. If possible, check your manual for details.
Try Pouring Some Gas into the Cylinder
- 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.
- 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.
- A less messy option, if you have a section of fuel or narrow gauge air line, is to dip it in gas to a depth of about 2 or 3 inches, put your thumb over the top, stick the end of the line into the plug hole, and take away your thumb, allowing the gas to run out.
Empty Fuel Lines Can Cause Difficult Starting
If you run the engine until it cuts out before re-filling, the fuel lines will have thoroughly emptied out. Remember you need to push the primer button 5 to 10 times before starting the engine again. If the check valve in the primer button isn't the best, this can cause difficult starting. If the button is working properly, it should feel "spongy".
Keep pressing it until it fills with gas and most of the air bubbles are gone. You can check whether the primer is working properly by partially filling the tank with gas but allowing the end of the line without the filter to remain above the surface (the line with the filter should be submerged). When you repeatedly push the primer button, gas should flow out of the line without the filter.
Maybe There's a Problem With the Spark Plug?
- Remove the plug with a spark wrench.
- 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.
- 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!
- A healthy spark should be blue. It might be difficult to see the spark in bright sunlight, so move the trimmer into the shade or indoors. If there's no sign of a spark, try a new one or even a plug from a lawn mower (to test the ignition circuit is okay) before you attempt any more in depth troubleshooting. (Don't use a plug which is longer than the original one as it could hit the piston!)
Can a Dirty or Cracked Spark Plug Prevent Starting?
If the plug won't spark, there are two possible causes:
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
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.
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.
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.
Multimeters from Amazon
Useful for checking voltage, current, continuity, and fuses. You can which will do the job adequately, or alternatively buy a cheap meter like Innova 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. a more expensive model like the Fluke 113
What About the Cap and Check Valve on the Gas Tank?
Unlike lawn mowers and other such yard equipment which is normally used in one orientation, trimmers, chainsaws, and hedge cutters must be used at all sorts of angles.
A lawn mower has a vent in the gas tank cap to allow air into the tank as gas leaves it. If the tank wasn't vented to atmospheric pressure, this would restrict the flow of gas out of the tank. (Turn a large soda bottle filled with water upside down and you'll see what I mean).
55 gallon oil drums also have a secondary bung which needs to be loosened to vent the drum, enabling smooth flow from the main outlet during emptying. On a lawn mower, the vent is just a hole with a fiber or plastic baffle inside the cap to stop gas splashing out. This isn't good enough on a trimmer if the tank is turned upside down, so a one way valve (check valve) is used.
This valve may be fitted in the cap of the gas tank or on the body of the tank itself.
The valve allows air in but gas can't leak back out. This could get clogged with dirt, so try starting the engine with the cap slightly loosened (keep the tank upright) to see if it makes any difference.
To clean the valve, gently slide a small flat blade jeweler's screwdriver into it to make sure its clear (make sure it's small diameter to prevent damage). I couldn't get access to the back of this valve because a fuel filter was pushed into place. There wasn't a problem with the valve so I didn't go any further.
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).
Now It's Time to Remove the Carburetor From the Engine
Your carburetor may be somewhat different from the one pictured, but the basic principle is the same.
- A section of membrane in the carburetor acts as a pump. Pulses of low and high pressure from the engine crankcase move this membrane backwards and forwards, and this sucks gas from the tank to a reservoir pocket in the carburetor.
- Reed valves act as check/one way valves to stop gas flowing backwards.
- A needle/metering valve operated by a diaphragm regulates flow into the reservoir pocket and shuts off flow when the pocket is full.
- A primer bulb is included which helps to suck fuel up into the carburetor before starting. Without this, the starter cord would have to be pulled multiple times to fill the carburetor with gas. Some carburetors don't have this primer button arrangement.
- It's a good idea to lay out parts on a piece of cloth or old towel to stop them rolling about during dismantling. You can also store them in a magnetic parts tray.
- Don't work outdoors like I did while disassembling. This was only to facilitate taking photos in good lighting conditions. If you drop small parts on a lawn, you will probably never find them!
- If you have an air compressor and blowgun, blow away any grime form the outside of the carburetor before removing. Do this before removing the air filter and fuel lines to avoid blowing grime into the carburetor or bursting the diaphragm inside it.
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.
Remove the Air Filter Housing
Two nuts hold this housing in place.
Never attempt to use a compressor blowgun to blow into any orifices/tubes before disassembling the carburetor. You may burst the diaphragm and gaskets!
Be Careful With a Blowgun
You can try cleaning surfaces/orifices with a blowgun, but never blow into any sealed compartments before disassembly to avoid rupturing the diaphragm/pump.
It's probably okay to blow over the outer surface of the assembled carburetor to remove grime, but keep the nozzle of the blowgun away from any openings.
Also remove any screen filters and loose parts first which could get blown away.
Wear safety glasses to protect your eyes from grime and small parts which could get blown up towards your face.
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!
Check the Primer Bulb for Holes
- The primer on a two-stroke engine is used for sucking fuel up into the carburetor. Without the primer bulb, the starter cord would have to be pulled loads of times before the fuel lines and carburetor fill up.
- Holes or splits can eventually form in the primer bulb. If this happens when the bulb is full, you'll probably notice it because gas will leak out. However, if it has been some time since the trimmer was used and the bulb has emptied out, any holes will likely prevent it sucking fuel up from the tank.
- If the material of bulb has deteriorated, it's possible also that it won't seal properly against the body of the carburetor, resulting in an air leak. Again, this will prevent the bulb sucking up fuel.
- A check or non return valve is part of the priming system (the red piece shown in the photos). This has two one-way sections. When you push the primer bulb, fuel flows from the bulb through the central one-way section to the carburetor (which, according to Wikipedia, is a duck bill valve). Fuel can't flow back because pressure squeezes the duck bill closed. Dirt in this valve can keep it open, allowing fuel to flow back to the primer bulb, so the carburetor doesn't get properly primed.
- The outer section is known as an umbrella valve and the floppy outer edges prevent fuel flowing backwards. When you release the bulb, suction causes the flexible edges of the valve to rise and fuel enters the bulb. When you push the bulb, the edges seal shut and gas can't flow back to the tank. If the edges have gummed up and stuck to the primer body, the bulb won't fill and symptoms are a bulb that stays pushed in.
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:
- 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.
- Fuel flows down passageway at point 2.
- Fuel flows back up at point 3. The reed valve prevents fuel flowing back.
- 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.
- 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).
- 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).
Fuel is pumped by suction from the crankcase acting on a membrane. The pulses of pressure move the membrane backwards and forwards, sucking fuel. Reed valves prevent fuel flowing backwards. Handle this piece with care as it can be easily damaged.
Note: In the photo, this appears to be one piece. However if you zoom in, you can see that there are in fact two parts stuck together. Firstly a gasket (which makes contact with the numbered part of the carb) and secondly a part which has the valves and pump. When reassembling, it's important to replace these in the correct order to prevent leaks.
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.
Diaphragm on a Two-Stroke Engine Carburetor
Lawn mowers and other similar engines often have a float bowl and float. This acts in a similar way to a toilet cistern and ball cock valve, ensuring there is always a full reservoir of fuel at a constant level from which the jet is supplied with fuel. This is important for the speed behavior of the engine.
Unfortunately, a float bowl and float rely on gravity for correct operation and this system can't be used on a trimmer which is used in all orientations.
- Instead, a reservoir compartment is built into the carburetor. As this empties, external atmospheric pressure acting on the diaphragm pushes the centre of it inwards. This forces a metering lever/rocker arm to pivot, lifting a needle valve up out of its seat, allowing fuel to flow into the reservoir pocket. Once the pocket fills, the process is reversed and the valve closes. So the pocket is always kept filled with fuel.
- Just like the reed valve piece, the diaphragm is delicate and needs to be separated carefully from the body of the carburetor, especially if it has become stuck.
- Over time, the diaphragm can become punctured or get stretched and "baggy" in which case the needle valve can stay shut, even when the reservoir pocket is empty. The material can also stiffen, preventing proper operation.
Clean All Surfaces
Clean all surfaces and fuel passageways with a carburetor cleaning aerosol. This removes any gum deposits which can clog pathways and jets.
If you have a compressed air blow gun, you can use it to aid drying. Set parts aside to dry once cleaned. Plastic parts, especially the reed valve piece and diaphragm, should be dried immediately after cleaning as the solvents in carburetor cleaner can be harsh and dissolve the plastic. Using IPA to clean these parts and the metering needle might be a safer option to avoid damage.
Avoid bending the reed valves with the force of the jet by spraying gently or resting the piece on a flat surface.
If you find your trimmer leaks fuel when not in use, the problem can be caused by a damaged tip on the metering needle or an accumulation of gum. This can be removed with a cotton bud soaked in isopropyl alcohol (IPA).
Re-Assembling the Carburetor and How to Reconnect Fuel Lines
Once everything has dried, re-assemble the carburetor.
- Sandwich the gaskets and diaphragm between the various sections of the carburetor, replace all screws and only barely tighten.
- 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.
- 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.
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.
Adjusting the Idle Speed
If the engine keeps cutting out when idling, the idle speed may be too low.
- Allow the engine to run at top speed for about a minute, then release the trigger.
- Turn the idle speed screw clockwise about one eight of a turn and wait to see if the engine continues to run without cutting out.
- If it still cuts out, turn the screw another eight of a turn.
- Some trimmers and all hedge cutters have clutches which prevent the cutting head from operating when the engine is idling. If the blade starts moving or the trimmer line starts spinning, turn the screw slowly back counterclockwise until movement stops.
What If a Trimmer Won't Stop
If the start/stop switch fails on your trimmer (or other engine), turn on the choke. This will flood the engine and stop it promptly.
Trimmer Cuts Out on Full Throttle
- Check the vent in the cap/tank isn't blocked.
- Ensure cracks in intake fuel line in tank aren't sucking in air, preventing fuel from getting to carburetor.
- Make sure choke is off.
- Are you using the correct mixture? Use 1:50 or 1:40 oil:gas. Too much oil can cause problems.
- Clean air filter and exhaust screen.
- Diaphragm/pump section in carburetor may have stiffened so it can't pump/meter fuel.
Did You Get Your Engine Started Using the Information in This Article?
Questions & Answers
© 2014 Eugene Brennan