This page is from "SplitFires" web site..

Plug Tips Tell The Story   Looking at spark plug firing tips can tell you if your engine has a problem that needs correcting. They reflect the performance of a well tuned engine or a poorly maintained one. Bad spark plugs can cause overheating, rough running, power loss and even engine failure. Check your spark plugs regularly and use this chart as a guide to spot performance problems. 1. Normal 2. Normal with Red Coating      Cause: Red coating resulting from fuel       additive in unleaded fuel. 3. Fuel Fouled      Cause: Faulty choke-overly rich fuel       mixture ignition problems or plug heat       range too cold. 4. Detonation      Cause: Over-advanced timing, low       octane fuel. 5. Worn Plug      Cause: Plug is used beyond its service       life. Need to replace. 6. Carbon Fouled      Cause: Plug heat range too       cold-extended low speed driving. 7. Pre-Ignition      Cause: Plug heat range too hot-over       advanced timing, air/fuel mixture       too lean. 8. Oil Ash Fouled      Cause: Poor oil control-worn rings,       worn valve guides and valve seals. 9. Mechanical Damage