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tan colored plug means that the engine is running normal and the
air/fuel mixture is correct. This is the correct color a spark plug
should be and it tells the mechanic everything is fine with the
engine. The mechanic would install a new properly gapped plug. When
installing a new plug, replace the old one with the same heat range.
This plug shows normal wear in the center electrode. A new plug
would have square edges that helps the plug fire better. |
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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.
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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 spark 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).
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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.
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| 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. |
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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. |
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| This
plug 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. |
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This
plug 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
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| This
plug 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. |
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