Inspecting Spark Plugs

Figure 2-48.—Carbon fouled. Figure 2-49.—Oil fouled. 2-39 ASH FOULED (fig. 2-50) appears as red, brown, yellow, or white colored deposits which accumulate on the insulator, resulting from poor fuel quality or oil entering the cylinder. Most ash deposits have no adverse effect on the operation of the spark plug as long as they remain in a powdery state. However, under certain conditions these deposits melt and form a shiny glaze on the insulator which, when hot, acts as a good electrical conductor. This allows current to follow the deposit instead ofjumping the gap, thus shorting out the spark plug. Spark plugs, having a powdery condition, may be cleaned, regapped, and replaced. Those having a glazed deposit are to be replaced. PREIGNITON DAMAGE (fig. 2-5 1) appears as burned or blistered insulator tips and badly worn Figure 2-50.—Ash fouled. electrodes, resulting from over-advanced timing, low- octane fuel, wrong spark plug heat range (too high), or a lean air-fuel mixture. Spark plugs, having this condition, are to be replaced with ones having the recommended heat range. When a spark plug is removed for cleaning or inspection, it should be regapped by the engine manufacturer’s specifications. New spark plugs are also to be regapped before installation, as they may have been dropped or mishandled and are not within specifications. A wire type feeler gauge should be used to measure spark plug gap. Slide the feeler gauge between the electrodes. If needed, bend the side electrode until the feeler gauge fits snugly. The gauge should drag Figure 2-51.—Preignition damage.