The spark plug is always a trustfull advisor about the engine΄s tuning and general condition. Here are some examples:
NORMAL : Brown, grey or gold/yellow color of the insulator suggests good engine condition/carburator adjustment.
WET OIL FOULED : While this is possible with an excessively cold plug or rich AFR (Air-Fuel ratio), a wet-fouled plug like this, is more likely the result of oil or some other contaminant being introduced into the combustion chamber. The smell and appearance of the liquid on the plug would be a key indication of what the problem is.
DRY CARBON FOULED : This is a common result in use of a colder heat range spark plug in an engine that doesn’t operate frequently at higher temperatures. If this was a spark plug rated at the factory heat range, other problems could be an overly-rich AFR, electrical problem causing misfires or extended operation of little to no throttle load.
MELTED ELECTRODE : This is what eventually happens to plugs that face either extreme or extended conditions that cause a plug to speckle. The excessive heat literally melts the metal and the violent detonation breaks away the ceramic insulator. If a spark plug is pulled out of an engine looking like this, expect to have more engine parts needing replacement.
DEPOSITS + CRACKED INSULATOR : Spark plugs peppered with deposits indicates oil or some other contaminant entering the combustion chamber. Instead of fouling out the plug, the liquid material evaporates and leaves behind crystal-like particles capable of deteriorating the plug and engine internals quickly.
OVERHEATED METAL SPECKLING : Seeing this condition is the first sign of too much heat on the spark plug. Before assuming a step colder plug is the only solution needed, make sure the engine itself isn’t overheating, the AFR isn’t too learn and that the timing isn’t over-advanced.