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S
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APPEARANCE
NORMAL
Light brown, tan or grey firing end.
DRY AND WET FOULING
Fouling, either dry (top - matt black, sooty) or
wet (bottom - gloss black, sticky),
OVERHEATING
When overheating occurs, deposits which
have accumulated on the insulator tip may
melt and give the tip a glazed appearance
DEPOSITS
Insulator nose and electrodes encrusted with
a build of deposits - usually off white in color.
LEAD FOULING
Lead deposits on the insulator nose. These
are usually a yellowish brown in color.
BREAKAGE
Physical damage to the insulator nose.
NORMAL LIFE
Growth of the plug gap during a plug's
working life is normal. However, the
increased gap will mean the spark is less
efficient and hence fuel is wasted and strain
is put on the ignition system.
ABNORMAL EROSION
This is accelerated growth of the gap
MELTING
The electrode surface will probably appear
lustrous and uneven.
EROSION, COROSION, OXIDATION
The surfaces of the electrodes are rough, in
extreme circumstances the electrode material
will have oxidized to the point of turning
green
LEAD EROSION
The ground electrode will appear worn away,
the central electrode will appear chipped and
the insulator nose will take on a yellowish
brown.
Multiquip Inc. ◦ GA Series Generators ◦ Manual No. GAGENSM
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A good indicator that the plug is functioning correctly and general
engine conditions are good.
Can be caused by many different conditions. Carbon deposits build up
when the plug fails to fire correctly and burn them off. Air/fuel mixture
too rich, choke stuck on, electrical problem, extended periods of low
speeds, plug heat range too cold. All should be investigated.
Possible causes are over advanced ignition timing, air/fuel mixture too
lean, water or oil level too low, plugs not fitted (tightened) correctly,
plug heat range too hot.
This is often caused by oil leakage through the piston rings or valve
seals. Could be due to the wrong viscosity of oil being used.
Lead content of petrol used is too high. Try petrol with a lower lead
content.
Usually caused by abnormal thermal expansion in the combustion
chamber. Maybe thermal heating or cooling shock. Causes as for
overheating above.
Plugs are at the end of serviceable life. Replace plugs as a set.
Due to the effects of corrosion, oxidation and reaction with the lead in
petrol.
Due to excessively high temperatures in the combustion chamber.
Causes as for overheating above.
Possibly due to age, engine standing for a long time without use.
This is due to chemical reaction between the nickel alloy electrodes
and the lead compounds in petrol.
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POSSIBLE CAUSE
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