One of the best things my father taught me was engine maintenance. My dad would examine ever spark plug he ever removed from any engine we had on our farm. It turns out that the spark plug is telling the story of your engine.
The color of your spark plug is a valuable piece of information about how well you ATV is really running. By examining the insulator firing nose color, a mechanic can determine a great deal about your ATV’s engines overall operating condition.
A light tan/gray color tells you that the spark plug is operating at optimum temperature and that the engine is in good condition. Dark coloring or heavy black wet or dry deposits can indicate rich condition, too cold a heat range spark plug, a possible vacuum leak, low compression, overly retarded timing or too large a plug gap and for us using fuel controllers whether or not our fuel map is correct.
A wet spark plug is usually an indication of a leaking head gasket, leaking rings or valve-train issues or an extremely rich condition. The nature of the liquid on the firing tip will help you diagnose your problem.
Signs of fouling or excessive heat must be quickly traced to prevent further deterioration of performance and ultimately possible engine damage.
NGK Spark Plugs have a FAQ page that answers many of the spark plug questions.