Figure 2-48.Carbon fouled.
Figure 2-49.Oil fouled.
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
manufacturers specifications. New spark plugs are
also to be regapped before installation, as they may
have been dropped or mishandled and are not within
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.