By ?????????Look at the spark plug porcelain 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
porcelain 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. The plug
below shows normal wear in the center electrode. A new plug
would have square edges that helps the plug fire better.
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 spar 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 porcelain 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 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*
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.
