How to Troubleshoot a Bolens String Trimmer

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You can troubleshoot common problems with your string trimmer.
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Bolens string trimmers are available at most big-box stores, and you can also buy one online. They are gas-powered trimmers, and models are available with curved and straight shafts.


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If you have a Bolens string trimmer and it's malfunctioning, you can follow the same troubleshooting procedure you would follow for most other brands. Common problems include hard starting, rough running or failure of the trimmer head to spin. You can't fix everything yourself, but if you're handy with two-cycle gas engines, you should be able to handle most problems that arise.

The String Trimmer Won't Start

A gas engine needs fuel, air and a spark, and the easiest place to start troubleshooting is with the last two. Make sure the trimmer has enough air by removing and cleaning or replacing the air filter and then removing and cleaning the spark arrestor, which is a small screen placed behind the muffler to prevent sparks from exiting the machine. You can usually clean this with a wire brush.


If the engine still won't start, remove the spark plug and clean the terminals or replace the plug if the terminals are caked with oil or gunk. You should also replace the plug if the terminals are eroded, and the gap is wider than that specified by the trimmer manufacturer. You can check the gap with a gap gauge.

Continued failure of the engine to start points to a problem with the fuel. The fuel filter may be blocked, so remove it from the gas tank and replace it. It's also possible that the engine has been burning old fuel and the carburetor is gummed up. You may be able to clean the carburetor by removing the air filter, spraying in one or two squirts of carburetor cleaning fluid and attempting to start the trimmer. If that doesn't work, you may have to remove the carburetor and clean or replace it.


The Trimmer Runs Erratically

If the trimmer starts but stalls immediately, the problem could be a blocked air filter, fuel filter, or spark arrestor. The carburetor may also be gummed up and in need of cleaning. These conditions are usually responsible if the trimmer keeps running but stutters and fails to reach full performance.

If you're sure that the carburetor and all the filters are clean, you may need to adjust the carburetor. There are two adjustment screws on a Bolens carburetor instead of the three you usually find on a trimmer carburetor. They are located next to the carburetor, and the one closest to you is the one that probably needs adjusting using a special carburetor adjustment screwdriver.


Turn this screw counterclockwise about a quarter turn and then start the trimmer. It should run more smoothly, but if it begins to stall, turn the screw back or forth as needed until the engine runs smoothly.

Troubleshooting the Trimmer Head

When the engine is working normally but the trimmer head won't spin, the problem can usually be traced to the clutch or the driveshaft. Remove the trimmer head from the driveshaft, check the notches on the end of the shaft and replace the shaft if the notches are worn and smooth. You should replace the trimmer head at the same time. Although it's a long shot, you don't want the trimmer to start up accidentally while you're working on it, so be sure to remove the spark plug wire.


If the driveshaft appears OK, the clutch is probably at fault. You may need a little experience with small engines to disassemble the motor housing to replace it, but the job isn't difficult, and a new clutch is inexpensive. Just be sure to follow the service manual instructions to make sure you do the job correctly.