Carbon fouled
Soft, dry, sooty deposits covering the entire firing
end of the plug are evidence of incomplete combus-
tion. Even though the firing end of the plug is dry,
the deposits decrease the plug’s insulation. The car-
bon forms an electrical path that bypasses the elec-
trodes resulting in a misfire. One or more of the
following conditions can cause carbon fouling:
1. Air/fuel mixture too rich.
2. Spark plug heat range too cold.
3. Clogged air filter.
4. Improperly operating ignition component.
5. Ignition component failure.
6. Low engine compression.
7. Prolonged idling.
Oil fouled
An oil fouled plug has a black insulator tip, a
damp oily film over the firing end and a carbon
layer over the entire nose. The electrodes are not
worn. Common causes for this condition are:
1. Incorrect air/fuel mixture.
2. Low idle speed or prolonged idling.
3. Ignition component failure.
4. Spark plug heat range too cold.
5. Engine still being broken in.
6. Valve guides worn.
7. Piston rings worn or broken.
Oil fouled spark plugs may be cleaned in an
emergency, but it is better to replace them. It is im-
portant to correct the cause of fouling before the en-
gine is returned to service.
Gap bridging
Plugs with this condition have deposits building
up between the electrodes. The deposits reduce the
gap and eventually close it entirely. If this condition
is encountered, check for excessive carbon or oil in
the combustion chamber. Be sure to locate and cor-
rect the cause of this condition.
Overheating
Badly worn electrodes and premature gap wear
are signs of overheating, along with a gray or white
LUBRICATION, MAINTENANCE AND TUNE-UP
65
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25
SPARK PLUG CONDITIONS
Normal
Carbon fouled
Oil fouled
Sustained preignition
Overheated
Gap bridged