8.2
Section 8
Electrical System and Components
Inspection
Inspect the spark plug when it is removed from the
cylinder head. The deposits on the tip are an
indication of the general condition of the piston rings,
valves, and carburetor.
Normal and fouled plugs are shown in the following
photos.
Normal: A plug taken from an engine operating under
normal conditions will have light tan or gray colored
deposits. If the center electrode is not worn, a plug in
this condition could be regapped and reused.
Carbon Fouled: Soft, sooty, black deposits indicate
incomplete combustion. Incomplete combustion is
usually caused by overrich carburetion, weak ignition,
or poor compression.
Worn: On a worn plug, the center electrode will be
rounded and the gap will be eroded .010" or more
than the correct gap.
Wet Fouled: A wet plug is caused by excess fuel, or oil
in the combustion chamber. Excess fuel could be
caused by operating the engine with too much choke.
Oil in the combustion chamber is usually caused by
worn piston rings or valve guides.
Chalky White Deposits: Chalky white colored
deposits indicate overheating. This condition is
usually accompanied by excessive gap erosion. A
clogged grass screen, clogged cooling fins, and lean
carburetion are some causes of overheating.
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