Chipped Insulator
A chipped insulator usually results from careless plug regapping. Under certain conditions, severe detonation also can split the
insulator firing ends. Replace the spark plugs.
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Wet Fouling (Oil Deposits)
The plug becomes shorted by excessive oil entering the combustion chamber, usually in the engine with many hours of
operation. Worn piston rings, cylinder walls, valve guides or valve stem seals are causes of oil entering the combustion
chamber. Only engine repairs will permanently relieve oil wet fouling.
IMPORTANT: New engines or recently overhauled engines may wet foul the plugs before normal oil control is achieved with
proper break‑in procedures. Such fouled plugs may be serviced (clean, file, and regap) and reinstalled.
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Cold Fouling
Dry, black deposits indicate a rich fuel mixture or a weak ignition. A clogged flame arrestor, a flooding engine, a sticky choke, or
weak ignition components all are probable causes. If, however, only one or two plugs in the set are fouled, check for sticking
valves or bad ignition leads. After correcting the cause, service (clean, file, and regap) plugs and reinstall.
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Engine Troubleshooting
90-8M0099748 eng DECEMBER 2015 © 2016 Mercury Marine Page 3C-5
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