11 Things Your Spark Plugs Are Telling You

“One of the best things my father taught me was engine maintenance. My dad would examine ever spark plug he ever removed from any engine we had on our farm. It turns out that the spark plug is telling the story of your engine.”

The color of your spark plug is a valuable piece of information about how well you ATV is really running.  By examining the insulator firing nose color, a mechanic can determine a great deal about your ATVโ€™s engines overall operating condition.

A light tan/gray color tells you that the spark plug is operating at optimum temperature and that the engine is in good condition.  Dark coloring or heavy black wet or dry deposits can indicate rich condition, too cold a heat range spark plug, a possible vacuum leak, low compression, overly retarded timing or too large a plug gap and for us using fuel controllers whether or not our fuel map is correct.

A wet spark plug is usually an indication of a leaking head gasket, leaking rings or valve-train issues or an extremely rich condition. The nature of the liquid on the firing tip will help you diagnose your problem.

Signs of fouling or excessive heat must be quickly traced to prevent further deterioration of performance and ultimately possible engine damage.

spark-plug-normalNormal Condition
An engineโ€™s condition can be judged by the appearance of the spark plugโ€™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.

spark-plug-dry-wet-foulingDry 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 engineโ€™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 370-470ยฐC (700-790ยฐF) 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 โ€“ 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 1,200~1,300ยฐC (2,200~2,400ยฐF).

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.

NGK Spark Plugs have a FAQ page that answers many of the spark plug questions.

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