Mercury 305 CID, Service Manual, Page: 64 / 967

1C-4 - TROUBLESHOOTING 90-823225--1 1096
Pre-Ignition Damage
Pre-ignition damage is caused by excessive high
temperatures. Center electrode melts first, followed
by ground electrode. Normally, insulators are white
but may be dirty if plug has been misfiring. Check for
correct plug heat range, advanced ignition timing,
lean fuel mixture, incorrect fuel used, malfunctioning
cooling system, leaking intake manifold or lack of lu-
brication.
72422
Reversed Coil Polarity
Concave erosion of ground electrode is an indication
of reversed polarity. Center electrode will show only
normal wear. Engine will misfire and idle rough. To
correct, reverse primary coil leads. Replace spark
plugs.
72422
Splashed Deposits
Spotted deposits, which sometimes occur after long
delayed tune-up, accumulate after a long period of
misfiring. When normal combustion temperatures
are restored, upon installation of new plugs, deposits
loosen from top of piston and head and are thrown
against hot insulator. Clean and service plugs and re-
install.
72423
Mechanical Damage
Mechanical damage to spark plug firing end is
caused by foreign object in combustion chamber. Be-
cause of valve overlap, small objects can travel from
one cylinder to another. Check all cylinders, intake
manifold and exhaust material to prevent further
damage.
72423
IMPORTANT: When working on engine, spark
plug holes and carburetor throat should be kept
covered to prevent foreign objects from entering
combustion chamber.