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spark plug reading

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ok...i have searched but can't really find an answer to this question.....say i am trying to jet my bike...i take a full throttle run and it is really rich and the plug is black/brown...now if i lean it out and do another run will it take the brown color off the plug and turn it white or do i need another new plug???


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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.


Dry and 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.



When a spark plug overheats, deposits that have accumulated on the insulator tip melt and give the insulator tip a glazed or glossy appearance.



The accumulation of deposits on the firing end is influenced by oil leakage, fuel quality and the engine's operating duration.


Lead Fouling

Lead fouling usually appears as yellowish brown deposits on the insulator nose. This can not be detected by a resitsance 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.



Breakage is usually caused by thermal expansion and thermal shock due to sudden heating or cooling.


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)


Abnormal Erosion

Abnormal electrode erosion is caused by the effects of corrosion, oxidation and reaction with lead - all resulting in abnormal gap growth.



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).


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.


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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ya i understand how to read the plug but what i want to know is if a lean condition will turn a black plug white again

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YOU do not need A new plug every time you check your jetting, infact A used plug with color to it will give A better reading. Now this dosen't apply to A oil fouled plug.

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Using a used plug with not give you the most accurate reading. It will be a minor mix of previous problems going on. If you are after the most accurate reading, your best bet is to use new plugs.

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When you get A light build up on the plug for color reading, then go out and retest your not putting A layer over old, what you are doing is chaging the color of the soot build up, when you get jetted right and your motor is 1100 to 1200 degrees at the plug it will turn brown, when you start getting up to 1300 degrees every thing will turn white because it will start to melt all the soot off. If you install A new plug and your jetting is right on it may take A while to see any color because these no build up to color, the plug is too clean.

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