Symptoms
- Runs fine at idle but bogs when cutting
- Loses power in tall or thick grass
- Engine sounds strained then dies
- Better performance right after cleaning air filter
Likely causes
- CommonDirty air filterRestricted air makes the engine run lean and lose power under load.
- CommonCarburetor mixture set too leanThe high-speed adjustment screw may need richening for heavy loads.
- PossibleClogged spark arrestor screenA carbon-clogged screen restricts exhaust flow under load.
- PossibleOld or wrong fuel mix ratioToo little oil or stale gas reduces power and lubrication.
- Less commonDull or wrong-size trimmer lineOversized or old brittle line increases drag on the motor.
What to do
- Clean or replace the air filter first, since this is the most common cause.
- Drain old fuel and mix fresh gas at the manufacturer's specified ratio.
- Check the muffler's spark arrestor screen and clean off carbon buildup.
- If the trimmer has adjustable carburetor screws, follow the owner's manual to slightly richen the high-speed mixture.
- Switch to the manufacturer-recommended line diameter if you've been using thicker line.
- Avoid cutting extremely thick or wet grass in one pass; take multiple lighter passes instead.
- If bogging continues after these checks, have the carburetor professionally tuned.
Tools and parts
- Air filter
- Small screwdriver
- Fresh fuel mix
Air filterSearch: “string trimmer air filter”
Spark arrestor screenSearch: “small engine muffler spark arrestor”
When to call a pro
If adjusting the filter and fuel doesn't help, a small-engine technician can properly tune the carburetor with a tachometer.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.