Full String Trimmer — Loses Power When Cutting guide
Use the interactive tool above for a personalized, step-by-step diagnosis — it asks one question at a time and takes you straight to the fix that matches your answers. Everything it can tell you is also written out below, in full, if you'd rather read through every possible cause first.
Safety notes
Before you begin
Keep hands and feet clear of the cutting head at all times, including during idle checks. Disconnect the spark plug wire for any fuel, filter, or muffler work.
Possible causes and how to fix them
Cutting load exceeds trimmer capacity
String trimmers are built for grass and light weeds. Thick or woody material demands far more power than the engine and cutting line are designed for — this isn't a fault, it's a mismatch between tool and job.
- For heavier growth, consider a brush-cutter blade attachment (if your trimmer supports one) instead of standard line.
- Work in shorter passes, letting the engine recover between cuts rather than pushing continuously through thick material.
- For genuinely woody brush, a different tool (brush cutter, hedge trimmer) may be more appropriate than a string trimmer.
Parts that may help: brush-cutter blade attachment
Clogged air filter
Under cutting load the engine needs more airflow than at idle — a partially clogged filter that seemed fine idling can starve the engine as soon as it's put under real load.
- Foam filters: wash in warm soapy water, air dry fully, lightly oil before reinstalling.
- Replace if torn or heavily saturated.
Parts that may help: engine-model-specific air filter, foam air filter oil
Clogged spark arrestor screen
A clogged spark arrestor restricts exhaust flow. Idle can be unaffected while cutting load — which demands far more exhaust flow — reveals the restriction as a loss of power.
- Remove the spark arrestor screen and clean with a wire brush, or replace if heavily caked.
- Reinstall securely — this part matters for fire safety.
Parts that may help: replacement spark arrestor screen
Carburetor main jet partially clogged
Deposits build in the main fuel jet over seasons, especially with ethanol fuel. A partial clog shows up specifically under load/high RPM, since that's when the main jet does the work.
- Remove the carburetor bowl and clean the main jet with carb cleaner spray and a strand of wire — never a drill bit.
- A full carb rebuild kit is worth it if it hasn't been serviced in several seasons.
Parts that may help: carburetor/choke cleaner spray, engine-model-specific carburetor rebuild kit
Common causes ruled out — needs deeper diagnosis
Cutting load, air filter, spark arrestor, and main jet are all ruled out. Remaining causes — worn piston rings reducing compression under load, or a clutch slipping — need more involved diagnosis.
- This is a good candidate for a local small engine shop, especially if compression or clutch wear is suspected.
If this doesn't resolve it, this is a good candidate for a local small engine shop rather than continued DIY diagnosis.