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Normal plug
A black dry fluffy colored plug is caused by
deposits from a carburetor that is running too
rich (too much gas), or excessive idling on some
engines. Black smoke coming from the exhaust is a
sign of a rich air/fuel mixture. The rich air/fuel
mixture must be repaired before installing a new
spark plug. Common causes for a rich air/fuel
mixture are:
dirty air filter.
air mixture screw or carburetor needs adjusting.
choke is sticking.
carburetor float height is out of adjustment or
float is sticking open.
A black wet glazed colored plug is caused by the
burned oil that is leaking past the piston rings
or valve guides and burning in the cylinder. This
engine will have blue smoke coming out of the
exhaust. It is a sign that the engine will need
rebuilding.
Two strokes that have this color plug are burning
the transmission oil from the crankcase. If a new
spark plug is installed it will foul and quit
working. Common causes for this problem are:
Worn piston and rings.
Worn cylinder bore.
Worn valve guides.
Crankshaft seals are bad or cases that need
lapping (if on a two stroke engine with a
transmission).
A white colored plug is caused by engine
overheating. Failure to repair this engine will
result in severe engine damage. Common causes for
this are:
Incorrect spark plug (too hot heat range).
Low octane fuel.
Timing is not set properly.
Cooling problems, (dirty cylinder fins, no or low
water if water cooled, low or no engine oil).
Carburetor air/fuel mixture is too lean (too much
air).
Leaking crankshaft seals, no oil, base or head
gasket leaks, or crankcase leaks on two stroke
engines.
The plug below shows what can happen when
something hits the spark plug. Something from
inside the engine has hit the plug and this
problem must be repaired before running the engine
further. Make sure the spark plug is the correct
length for the engine
Excessive detonation has caused the porcelin on
this plug to break away. If this engine is allowed
to run, engine damage can occur. Make sure the
fuel octane is high enough for the engines
requirements.
The plug below has carbon across the plug gap.
This is usually caused by carbon that is loosened
and it lodges between the gap causing the plug to
misfire.
The plug below was caused by preignition and
failure to repair this condition will cause engine
damage. Check for the following:
correct heat range plug
over advanced ignition timing
lean air/fuel mixture
cooling system
lack of lubrication
The plug below is worn out from being used for a
long period of time. Notice how the center
electrode is round and worn from use. A spark plug
that is worn takes a lot more voltage to fire and
can cause poor engine running.