How to Troubleshoot a Troy-Bilt 4-Cycle String Trimmer

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Follow some simple troubleshooting steps to help resolve string trimmer problems.
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A Troy-Bilt 4-cycle string trimmer makes easy work of cutting grass, trimming weeds, edging and making decorative trims around softscapes, hardscapes and other landscape structures. But if you hit a roadblock with your trimmer's smooth operation, following some simple troubleshooting steps may help you resolve the problem without having to call a small-engine mechanic. Your first go-to source is the Troy-Bilt owner's manual for your string trimmer model, which offers solutions to some common problems.


Troy-Bilt encourages string trimmer operators to heed all safety precautions listed in the user manual. Wear eye protection, such as safety glasses, goggles or eyeglasses, before examining, adjusting or making repairs. Switch the engine to the "off" position and remove the spark plug before maintaining or repairing your string trimmer.

Engine Starting and Idling Problems

Old fuel can be the reason that a string trimmer won't start, and the easy fix is to drain the old fuel and fill the fuel tank with new fuel. At the end of the season, always drain the leftover fuel before properly storing a string trimmer. Over winter, the old fuel will partially evaporate, and the sticky substance that's left behind can clog the carburetor. You may also have to use a carburetor cleaner when you drain the fuel to remove this sticky substance and possibly adjust the carburetor after cleaning it, following all directions in the user manual for making this adjustment.


Remove the air filter and inspect it; if it's clogged, replace it. Use a spark plug tester to determine if the spark plug is defective. You should see a strong spark between the tester terminals; if you don't or if you observe a cracked porcelain insulator or a blackened/burned electrode, it's time to replace the spark plug. Clean the spark arrestor — the small screen that prevents the engine from emitting sparks when you crank it — with a wire brush or replace it.

Cutting Line Improperly Advances

Without the proper operation of the cutting line — the "string" on a string trimmer — the primary function of this tool is a no-go. If your trimmer has seen a lot of action by cutting some serious weeds or other dense vegetation, particularly if the green matter was tall, the cutting attachment may be clogged or bound because of it. Making sure the engine is in the "Off" position, clear the cutting attachment by carefully removing all the grass and weeds from it. Clean the inner reel as well as the outer spool.


After cleaning this mechanism, you may see that the line is twisted. Disassemble the spool and unkink the line before rewinding it back into the spool. You may also see that the line has simply reached its end, so all you'll have to do is replace the inner reel with a new string-filled one. If there's only a short section exposed, not enough to trim weeds, push the bump knob until you see 4 inches of line outside the cutting attachment.

When replacing any parts on your Troy-Bilt string trimmer, only use replacement parts that are approved by the manufacturer as noted in the user manual. If you use non-approved parts, your trimmer may not properly function, and you may void the warranty. When in doubt about a replacement part or a repair that's outside your wheelhouse, call a factory-certified representative for guidance.



Victoria Lee Blackstone is a horticulturist and a professional writer who has authored research-based scientific/technical papers, horticultural articles, and magazine and newspaper columns. Her writing expertise covers diverse industries, including horticulture, home maintenance and DIY projects, banking, finance, law and tax. Blackstone has written more than 2,000 published works for newspapers, magazines, online publications and individual clients.