7 Reasons Why Your Gas-Powered String Trimmer Isn’t Starting

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Keeping tall grass and light brush trimmed along fencing or around the base of tree trunks (or those pink flamingos you dutifully install on the front lawn each summer) is an essential finishing touch following a lawn cutting. To accomplish this, many professional and do-it-yourself landscapers rely on gas-powered string trimmers. Like with all gas-powered lawn and garden equipment, if the engine won’t start, the string trimmer won’t be of much use. What should you do if your gas-powered string trimmer isn’t starting? Repair Clinic has identified seven likely reasons why the trimmer engine may not be starting along with the troubleshooting tips you can follow to fix the problem quickly.

Understanding how a gas-powered string trimmer engine works

To successfully troubleshoot why a gas-powered string trimmer isn’t starting, you’ll need to understand how a trimmer engine works. Not all string trimmers rely on a gas-powered engine (many trimmers are powered by a battery, a power cord, or even propane), but those trimmers that run on gasoline will use one of two different engine types.

Commercial trimmer engines vs. residential trimmer engines

Typically, commercial gas string trimmers utilize a four-cycle engine, while most residential string trimmers employ a two-cycle engine. While four-cycle engines will have a separate sump for oil, two-cycle engines will usually require the gasoline to be mixed with oil to provide the necessary lubrication for the engine to function smoothly. Since getting the right gas/oil mixture can be challenging, a product like TRUFUEL 50:1 Mix, which pre-mixes fuel and oil, can be poured directly into the string trimmer fuel tank.

The basics of string trimmer engine operation

To turn over the engine, the ignition switch must first be set to the start position. As the starter rope is pulled, the starter engages the drive cup attached to the flywheel, causing the crankshaft to rotate. The rotating crankshaft connects to a piston that moves vertically within the cylinder. When the piston descends, an intake port is exposed, allowing fuel, oil, and air to enter the cylinder. As the piston ascends, a vacuum is created, drawing the gas, oil, and air mixture through the carburetor and into the crankcase. The flywheel has permanent magnets installed in it, and as the magnets pass by the ignition coil, a magnetic field is generated. This magnetic field induces electricity, enabling the ignition coil to emit a high-voltage pulse to the spark plug. When the piston reaches the top of the cylinder, the spark plug ignites the compressed fuel and air mixture, propelling the piston back down. The spent fuel exits through an exhaust port, while fresh fuel enters the intake port, to continue the combustion process sustaining the engine’s operation.

Additional engine components that assist with starting

To assist with engine startup, particularly when it’s cold, string trimmer engines are equipped with either a choke, a primer bulb, or both. Pressing the primer bulb will draw extra fuel through the carburetor, while closing the choke will temporarily limit the airflow through the carburetor, creating a vacuum that will allow a higher amount of fuel to enter the cylinder.

How to troubleshoot a malfunctioning string trimmer engine

With all of these components working together to start and run the string trimmer’s engine, one malfunctioning part can prevent the string trimmer from starting. Here are the seven parts most likely to cause an engine starting problem as well as some troubleshooting advice on how to fix that problem:

1) Defective spark plug – Due to carbon build-up or a weakened electrode, a spark plug will degrade over time and be unable to ignite the compressed fuel and air inside the engine’s combustion chamber to start or run the trimmer engine. You can use an ignition tester to help determine the strength of the spark plug, but it’s probably easier to simply replace it with a new one, something we recommend doing as part of an annual string trimmer engine tune-up.

2) Restricted carburetor – Old or bad fuel can leave a gummy residue inside the carburetor creating a restriction. This will prevent the proper ratio of fuel and air from entering the cylinder which can cause the engine to have trouble starting. You should always make sure you keep fresh fuel in the tank and use a fuel stabilizer to help preserve its quality. You can try cleaning the carburetor ports with a dedicated carburetor cleaner, or WD-40, to clear out the restriction. If cleaning is not effective, you can often purchase a carburetor repair kit to replace some of the clogged components or install a new carburetor altogether.

3) Clogged fuel filter – That gummy residue that can clog a carburetor can do the same to the fuel filter located in the trimmer’s fuel tank. Simply replace the filter with a new one to solve the problem. Again, this is something that is recommended anyway as part of the engine’s annual maintenance.

4) Broken rewind spring – If the starter’s rewind spring is broken, the rope won’t be able to recoil onto the pulley resulting in the engine not starting. Many rewind springs can be replaced independently of the starter, but this can be tricky since the tightly-wound spring can easily become unwound during installation, something that happens with enough intensity to cause injury. As a safety precaution, it may be better to replace the entire recoil starter instead of just the spring.

5) Malfunctioning recoil starter – Other recoil starter components apart from the spring could be malfunctioning as well causing the starter to be unable to engage with the trimmer engine crankshaft. Inspect the starter rope, clutch, starter pawls, and, yes, the rewind spring for damage or wear and replace the component or components as necessary. Again, you may find it easier to simply install a brand-new recoil starter assembly to solve the problem.

6) Clogged spark arrestor – The spark arrestor is a thin metal mesh designed to prevent sparks emitted by the engine from exiting the muffler and potentially starting a fire. Over time the arrestor can become clogged with debris and affect engine performance. You can try cleaning the arrestor with a wire brush or replace the part with a new one.

7) Dirty air filter – By restricting the air coming into the carburetor, a dirty air filter can prevent the engine from starting as well. The air filter is yet another part that should always be replaced annually when tuning up the engine or whenever the filter appears visibly soiled.

Top Reasons String Trimmer Not Starting – String Trimmer Troubleshooting

Fix that string trimmer engine with genuine OEM parts

Your troubleshooting has identified that you need to install a new rewind spring to fix your string trimmer, but does it matter which spring you install? Yes! A successful repair requires that you install the exact original equipment manufacturer (OEM) part that matches your string trimmer model. By stocking only OEM parts, Repair Clinic assures you that you’ll be installing the right part. Enter the full model number of the string trimmer in Repair Clinic’s website search bar to view a comprehensive list of compatible parts, genuine OEM replacement parts from all the top string trimmer brands, including Craftsman, Echo, Husqvarna, Kawasaki, MTD, Poulan Pro, Ryobi, Toro, and Troy-bilt. You can then use the “Part Category” navigation filter (examples: “Starter”, “Carburetor”) along with the “Part Title” navigation filter (examples: “Rewind Spring”, “Float Needle”) to narrow the list down to the specific part or parts you need for your repair.

Free string trimmer repair help courtesy of Repair Clinic

You’ve located the replacement part you need; now you’re looking for some guidance on how to install the part. Repair Clinic has you covered by offering free string trimmer repair help in the “Videos & Articles” section of its website. Explore the vast collection of procedural videos, step-by-step guides, and equipment schematics to learn the proper way to install a rewind spring on an Echo Model GT225 String Trimmer or how to replace the carburetor on a Husqvarna Model 128CD String Trimmer. It’s all part of Repair Clinic’s commitment to being your repair partner.

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