Cold Fouling; Overheating; High Speed Glazing; Scavenger Deposits - MerCruiser GM4 Service Manual

Cold Fouling

Dry, black deposits indicate rich fuel mixture or
weak ignition. Clogged flame arrestor, flooding car-
buretor, sticky choke or weak ignition components
all are probable causes. If, however, 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.

Overheating

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 be-
ing used. Also check for over-advanced ignition
timing, cooling system malfunction, lean fuel/air
mixtures, leaking intake manifold or sticking valves.
Replace spark plugs.
Index
90-816462 2-695

High Speed Glazing

Insulator has yellowish, varnish-like color, indicating
that temperatures suddenly have risen, usually dur-
ing hard, fast acceleration under heavy load. Nor-
mal deposits do not get a chance to blow off. In-
stead, they melt and form a conductive coating.
Replace plugs. If condition recurs, use colder heat
range plug and service plugs more frequently.
72421

Scavenger Deposits

Powdery white or yellow deposits are built up on
shell, insulator and electrodes. This is normal ap-
pearance with certain branded fuels. Accumulation
on ground electrodes and shell areas may be un-
usually heavy, but may be easily chipped off. Plugs
can be serviced (clean, file and re-gap) and rein-
stalled.
72421
72421
72422
TROUBLESHOOTING - 1C- 3