Read your spark plug: the colour guide
Your plug's electrode is a free witness to what's happening in the combustion chamber. Remove it with the engine cold, look at the colour and texture in good light, then compare with the cases below. On a multi-cylinder engine, also compare the plugs with each other. Check the heat range and torque in your manual before refitting.
Light brown / tan
Healthy combustion: correct air-fuel mixture and ignition, good temperature. This is the look you want. Nothing to correct.
Dry, black and sooty
Mixture too rich (too much fuel), clogged air filter, mis-set idle, or too many short trips. The plug fouls and the spark weakens.
See the diagnosis:Black smoke from the exhaustIt uses too much fuelRough idle / the bike stalls at a stop
Black, wet and oily
Oil is getting into the chamber and burning: worn rings or valve stem seals on a 4-stroke, or too rich an oil mix on a 2-stroke.
See the diagnosis:Blue smoke from the exhaustExcessive oil consumption
White, glazed or blistered
Mixture too lean or temperature too high (wrong heat range, air leak). Danger: a white, melted plug warns of overheating that can wreck the engine.
See the diagnosis:The bike runs hot / overheatsLack of power / the bike won't pull like it used to
Wet with fuel
Flooded engine (too much choke/throttle) or no spark: the fuel isn't burning and soaks the plug. Common after a failed start.
See the diagnosis:It starts then stalls immediatelyThe bike won't start
A visual understanding aid â it does not replace a certified mechanic's diagnosis.
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