General Motors In-Line Engine

• Wet Fouling (Oil Deposits)
  Cause: Excessive oil entering combustion chamber, usually in engine with many hours of operation. Worn piston rings, cylinder walls, valve guides or valve stem seals are causes of oil entering combustion chamber.
  Fix: Only engine repairs will permanently relieve oil wet fouling. New engines or recently overhauled engines may wet foul plugs before normal oil control is achieved with proper break-in procedures. Such fouled plugs may be serviced (clean, file and re-gap) and reinstalled.
• Scavenger Deposits
  Cause: Powdery white or yellow deposits are built up on shell, insulator and electrodes. This is normal appearance with certain branded fuels. Accumulation on ground electrodes and shell areas may be unusually heavy, but may be easily chipped off.
  Fix: Plugs can be serviced (clean, file and re-gap) and reinstated.
• Overheating
  Cause: Insulator is dull white or gray and appears blistered. Electrodes are eroded and there is an absence of deposits. Check that correct plug heat range is being used. Also check for over-advanced ignition timing, cooling system malfunction, lean fuel/air mixtures, leaking intake manifold or sticking valves.
  Fix: Replace spark plugs.
• High Speed Glazing
  Cause: Insulator has yellowish, varnish-like color, indicating that temperatures suddenly have risen, usually during hard, fast acceleration under heavy load. Normal deposits do not get a chance to blow off. Instead, they melt and form a conductive coating.
  Fix: Replace plugs. If condition recurs, use colder heat range plug and service plugs more frequently.
• Cold Fouling
  Cause: Dry, black deposits indicate rich fuel mixture or weak ignition. Clogged flame arrestor, flooding carburetor, sticky choke or weak ignition components all are probable causes.
  Fix: If only one or two plugs in set are fouled, check for sticking valves or bad ignition leads. After correcting cause, service (clean, file and re-gap) plugs and reinstall.
• Chipped Insulator
  Cause: Careless plug re-gapping. Severe detonation also can split insulator firing ends.
  Fix: Plug must be replaced.

Compiled from service & troubleshooting documentation across every generator set that uses the General Motors In-Line.

Pooled from every document that names this exact engine — confirm the interval and part number against your unit's serial number.