Symptoms
- Hard starting or won't start at all
- Engine runs rough or misfires
- Poor power or fuel economy
- Plug tip looks black, oily, or heavily worn when removed
Likely causes
- CommonCarbon fouling from a rich mixture or short tripsExcess unburned fuel deposits carbon on the plug tip over time.
- CommonOil foulingOil reaching the combustion chamber, often from overfilling or engine wear, coats the plug and prevents good spark.
- PossibleWorn electrode gapThe gap widens with normal wear over a season or more, weakening the spark.
- Less commonWrong plug type installedAn incorrect plug can cause chronic fouling or poor performance.
What to do
- Disconnect the spark plug wire, then remove the plug with a spark plug socket.
- Inspect the tip: black and dry suggests a rich mixture, black and oily suggests oil fouling, white/gray is normal.
- Clean light fouling with a wire brush, or simply replace the plug since they're inexpensive.
- Check and set the gap to the specification in the owner's manual using a gap tool.
- Confirm you're using the correct plug type and heat range listed in the manual.
- Reinstall, hand-tighten then snug with a wrench, and reconnect the wire before testing.
Tools and parts
- Spark plug socket
- Gap tool
- Wire brush
Small engine spark plugSearch: “lawn mower spark plug”
When to call a pro
If a new plug keeps fouling quickly, an underlying oil-burning or rich-running issue needs to be diagnosed by a small engine shop.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.