Examining the tips of the spark plugs as they are removed can reveal a great deal about the health and performance of an engine. The appearance and color of the deposits can reveal other problems that may need fixing:
Normal deposits Light brown or tan colored.
Fuel fouled spark plug Black fluffy carbon deposits indicate an overly rich fuel mixture or possibly a weak spark. Check for such things as a stuck choke, a heavy or misadjusted carburetor float, a leaky needle valve in the carburetor, leaky injectors, low coil output or high resistance in the plug wires.
Wet spark plug A wet spark plug means the plug has not been firing. If not due to engine flooding, the problem may be a bad ignition cable (excessive resistance, shorted or arcing). But wet fouling can also be caused by dirt or moisture on the outside of the plug that provides a conductive path to ground, or by an internal crack in the ceramic insulator that shorts the plug to ground.
Oil fouled spark plug Heavy black deposits with an oily appearance. These are the result of oil entering in the combustion chamber, probably past worn valve guides, guide seals or rings. Switching to a hotter plug may help prolong plug life somewhat, but no spark plug will survive long under such conditions. The only permanent cure to this condition is to fix the oil consumption problem.