★ Whether or not the right temperature plug is being used can be ascertained by noting the condition of the ceramic insulator around the electrode A light brown color indicates the correct plug is being used. If the ceramic is black, it indicates that the plug is firing at too low a temperature, so the next hotter type should be used instead. If the ceramic is white, the plug is operating at too high a temperature and it should be replaced with the next colder type.
- Carbon Fouling [A]
- Oil Fouling [B]
- Normal Operation [C]
- Overheating [D)
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Caution!
- If the spark plug is replaced with a type other than the standard plug, make certain the replacement plug has the same thread pitch and reach (length of threaded portion) and the same type electrode (regular type or projected type) as the standard plug. If the plug reach is too short, carbon will build up on the plug hole threads in the cylinder head, causing overheating and making it very difficult to insert the correct spark plug later.
- If the reach is too long, carbon will build up on the exposed spark plug threads causing overheating, preignition, and possibly burning a hole in the piston top. In addition, it may be impossible to remove the plug without damaging the cylinder head.
Standard spark plug threads:
- Diameter: 10 mm
- Pitch: 1.0 mm
- Reach: 19 mm
Note. The heat range of the spark plug functions like a thermostat for the engine. Using the wrong type of spark plug can make the engine run too hot (resulting In engine damage) or too cold (with poor performance, misfiring, and stalling).
- Too short [A]
- Carbon builds up here [B]
- Correct reach [C]
