SER FAQ: LMFAQ: Checking the spark plug

S.E.R. FAQ

NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Checking the spark plug
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Checking the spark plug

Use the proper socket to remove the spark plug and inspect it for damage and general appearance:

Light gray or brown and smooth - this is the normal appearance. The mixture is correct and there likely no major problems with the engine.
Excessive black carbon - the mixture may be too rich or the spark plug may be the wrong type for your engine.
Damage to the electrodes - the mixture may be too lean, timing may be set incorrectly, or the spark plug may be the wrong type for your engine.

The best thing to do at this point is just replace it with a new spark plug and worry about the old one later. Actually, nearly every small engine maintenance book will recommend changing the spark plug every season anyhow.