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. 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 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.
