Normal Condition
An engine's condition can be judged by the appearance of the spark
plug's firing end. If the firing end of a spark plug is brown or light
gray, the condition can be judged to be good and the spark plug is
functioning optimally.
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Normal Life
A worn spark plug not only wastes fuel but also strains the whole ignition
system because the expanded gap (due to erosion) requires higher voltages. Normal
rates of gap growth are as follows:
Four Stroke Engines : 0.01˜0.02 mm/1,000 km (0.00063˜0.000126 inches/1,000 miles)
Two Stroke Engines : 0.02˜0.04 mm/1,000 km (0.000126˜0.00252 inches/1,000 miles)
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Overheating
When a spark plug overheats, deposits that have accumulated on the
insulator tip melt and give the insulator tip a glazed or glossy
appearance.
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Melting
Melting is caused by overheating. Mostly, the electrode surface is rather lustrous and
uneven. The melting point of nickel alloy is 1,200˜1,300°C (2,200˜2,400°F).
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Erosion, Corrosion and Oxidation
The material of the electrodes has oxidized, and when the oxidation is heavy it will
be green on the surface. The surface of the electrodes are also fretted and rough.
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Deposits
The accumulation of deposits on the firing end is influenced by oil
leakage, fuel quality and the engine's operating duration.
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Breakage
Breakage is usually caused by thermal expansion and thermal shock
due to sudden heating or cooling.
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Dry Wet Fouling
Although there are many different cases, if the insulation resistance
between the center electrode and the shell is over 10 ohms, the engine
can be started normally. If the insulation resistance drops to 0 ohms,
the firing end is fouled by either wet or dry carbon.
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Lead Fouled
Lead fouling usually appears as yellowish brown deposits on the
insulator nose. This can not be detected by a resistance tester at
room temperature. Lead compounds combine at different temperatures.
Those formed at 370-470°C (700-790°F) having the greatest
influence on lead resistance.
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Lead Erosion
Lead erosion is caused by lead compounds in the gasoline which react chemically with
the material of the electrodes (nickel alloy) as high temperatures; crystal of nickel
alloy fall off because of the lead compounds permeating and seperating the grain
boundary of the nickel alloy. Typical lead erosion causes the surface of the ground
electrode to become thinner, and the tip of the electrode looks as if it has been
chipped.
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Abnormal Erosion
Abnormal electrode erosion is caused by the effects of corrosion, oxidation and
reaction with lead - all resulting in abnormal gap growth.
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