Symptoms
- Hard to start, especially after sitting
- Engine runs rough or misfires
- Black, oily, or wet-looking deposits on the plug tip
- Runs better briefly after a fresh start attempt, then worsens
Likely causes
- CommonCarbon fouling from a rich fuel mixture or excessive choke useBlack, dry, sooty deposits on the plug tip indicate the engine is running richer than ideal.
- PossibleOil fouling from worn piston rings or over-oilingWet, oily deposits suggest oil getting into the combustion chamber.
- CommonOld plug simply worn outElectrodes erode over time and the gap can widen beyond spec, weakening the spark.
- PossibleIncorrect spark plug gapA gap that's too wide or narrow for the engine can cause misfires even on an otherwise good plug.
What to do
- Remove the spark plug and inspect its tip; black sooty deposits mean carbon fouling, while wet or oily deposits mean oil is getting into the cylinder.
- Clean minor carbon deposits with a wire brush or fine sandpaper, or simply replace the plug since they're inexpensive.
- Check the gap against the specification in your owner's manual using a gap gauge, and adjust if needed.
- If deposits are oily, check the oil level for overfilling and correct it.
- Address any underlying rich-running condition (like a stuck choke or dirty air filter) rather than just replacing the plug repeatedly.
- Reinstall the plug hand-tight, then snug with a wrench without overtightening, and reconnect the plug wire.
- If the plug fouls again quickly after replacement, an underlying fuel mixture or oil consumption issue needs to be addressed by a small-engine shop.
Tools and parts
- Spark plug socket or wrench
- Spark plug gap gauge
Generator spark plugSearch: “generator small engine spark plug”
When to call a pro
If a new plug fouls again quickly, have a small-engine shop diagnose the underlying rich-mixture or oil-consumption cause.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.