Wet, oily carbon deposits form an electrical leakage path along the insulator nose, resulting in a misfire. The cause may be badly worn engine or a malfunctioning ignition system.
A blistered white insulator or melted electrode indicates
over-advanced ignition timing or a malfunctioning cooling system.
If correction does not prove effective, try a colder grade plug.
A worn spark plug not only wastes fuel but also overloads the whole ignition system because the increased gap requires higher voltage to initiate the spark. This condition can also affect air pollution.
Next Section: 7. Final Inspection