Analyzing Spark Plugs

Remove the spark plug and inspect the working end. Its condition will help analyze your engine’s performance.

  • If the spark plug is coated with a light brown deposit, your engine is running properly. Scrape away the deposits with a knife and wire brush, and keep using the same plug. If you can’t easily remove the coating because it is too thick, replace the plug.
  • If the plug on four-cycle engines is black and oily, this usually means that the piston rings are worn and must be replaced—they are allowing oil from the crankcase into the combustion chamber. An oily plug on a two-cycle engine probably indicates too much oil in the oil-gas mixture. Check your owner’s manual and correct your mixing procedure.
  • If the plug is coated with a black powdery deposit, the gas-air mixture is too rich, or else the air filter is clogged. This carbon deposit can cause the plug to ground itself on the engine and spark improperly or not at all. Either clean or replace your air filter.
  • If the plug is coated with white or yellow deposits, a higher-test gasoline than necessary is being used. Check your manual for the correct type of gasoline to use. These deposits could also mean that there is too much air in the air-fuel mixture. If so, enrich the mixture by adjusting the carburetor. Turn the needle valve counterclockwise very slightly. See Adjusting Carburetors.
  • If the side electrode is pitted or has a small metallic mound on it, or if the center electrode has a round, worn appearance, this means that the electrodes are burned. Burning can also be indicated by a clean, white look. Burned electrodes are caused by overheating, which can result from any of three causes: (1) an air-fuel mixture that is too lean, (2) clogged cooling fins on the engine, or (3) incorrect engine timing.
  1. To correct the air-fuel mixture, adjust the needle valve on the carburetor (see Adjusting Carburetors).
  2. To clear the cooling fins, simply remove the shroud or cover from the flywheel, and clear the debris (e.g., grass or soil) from the fins with a small stick or probe.
  3. To correct the engine timing, check the points to make sure the plug is adjusted to the correct gap (see Tuning Gasoline Engines).

To remove the small metallic mound on the side electrode, file between the gap, using a very small file or thin carborundum paper (special sandpaper with carborundum as the abrasive). Then reset the points.