Symptoms
- New spark plug becomes black and sooty within a few uses
- Engine gets progressively harder to start over time
- Plug tip appears wet with fuel or oil
- Rough running that improves briefly after a plug change
Likely causes
- CommonToo much oil in the fuel mixExcess oil creates a rich condition that deposits carbon and oil on the plug faster than normal.
- CommonDirty air filter causing a rich mixtureRestricted airflow makes the engine run richer than it should.
- PossibleWrong spark plug heat rangeA plug that's too cold for the engine can foul from not burning off deposits.
- PossibleCarburetor set too richAn adjustable carburetor set incorrectly can consistently overfuel the engine.
- Less commonExcessive idling without loadLong idle periods don't burn off carbon buildup the way running under load does.
What to do
- Drain the fuel and remix using the exact ratio specified in the owner's manual, not a richer mix than recommended.
- Clean or replace the air filter to ensure proper airflow.
- Confirm you're using the correct spark plug model and heat range specified for your trimmer.
- Avoid letting the engine idle for extended periods without cutting; run it under normal working load instead.
- If your carburetor has adjustable mixture screws, check that it isn't set unusually rich.
- Install a fresh plug after making these corrections and monitor its condition after a normal session of use.
- If the new plug still fouls quickly, have a small-engine shop check the carburetor's mixture settings.
Tools and parts
- Spark plug wrench
- Fresh fuel mix
Spark plug (correct heat range)Search: “trimmer spark plug replacement”
Air filterSearch: “string trimmer air filter”
When to call a pro
If plug fouling continues after correcting the fuel mix and filter, have a small-engine shop check the carburetor mixture setting.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.