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Electrode gap check - use a wire type gauge for best results
Normal condition - A brown, tan or grey firing end indicates that the
engine is in good condition and that the plug type is correct
Carbon fouling - Dry, black sooty deposits leading to misfire and
weak spark. Caused by an over-rich fuel/air mixture, faulty choke
operation or blocked air filter
Overheating - A blistered white insulator and glazed electrodes.
Caused by ignition system fault, incorrect fuel, or cooling system fault
Electrode gap adjustment - bend the side electrode using the
correct tool
Ash deposits - Light brown deposits encrusted on the electrodes and
insulator, leading to misfire and hesitation. Caused by excessive
amounts of oil in the combustion chamber or poor quality fuel/oil
Oil fouling - Wet oily deposits leading to misfire and weak spark.
Caused by oil leakage past piston rings or valve guides (4-stroke engine),
or excess lubricant (2-stroke engine)
Worn plug - Worn electrodes will cause poor starting in damp or cold
weather and will also waste fuel
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