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Spark
Plugs and What They Say
Look
at the spark plug porcelin for the color and condition of
it. Plug colors tell the mechanic how the engine is running
and can be very important. Plugs change colors and the different
colors can explain such things as if the engine is running
too hot or if the engine is worn. The color should be read
by looking at the porcelin insulator and compared to the plugs
shown on the next few slides.
| A
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 here 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. |
| 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 pre-ignition 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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