Tech. Tip:How to"read" a spark plug?

Reading spark plugs can be a valuable tuning aid. By examining the Insulator firing nose color, an experienced engine tuner can determine much about the engine's overall operating condition.

In general, a light tan/gray color tells you that the spark plug is at the optimum temperature and that the engine is in good operating condition.

Normal Condition

Dark coloring, such as heavy black wet or dry deposits usually indicate a fouling problem. Heavy, dry deposits can indicate an overly rich condition, too cold a heat range spark plug, possible vacuum leak, low compression, overly retarded timing, or too large a plug gap.

Dry/Wet Fouling

Common causes for this problem are:

• Worn piston and rings.
• Worn cylinder bore.
• Worn valve guides.
• Crankshaft seals are bad or cases that need lapping (if on a two stroke engine with a transmission).

If the deposits are wet, it can be an indication of a breached head gasket, oil control from rings or valvetrain problems, or an extremely rich condition, depending on what liquid is present at the firing tip.

One must also look for signs of detonation, such as silver specs, black specs, or melting or breakage at the firing tip.

Overheating

Common causes for this are:

• Incorrect spark plug (too hot heat range).
• Low octane fuel.
• Timing is not set properly.
• Cooling problems, (dirty cylinder fins, no or low water if water cooled, low or no engine oil).
• Carburetor air/fuel mixture is too lean (too much air).
• Leaking crankshaft seals, no oil, base or head gasket leaks, or crankcase leaks on two stroke engines.

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

Here's a chart on how to read NGK sparkplug codes..

NGK SparkPlug Codes:

NGK code:  [B]  [P]  [6]  [E]  [S]

Tech Tips: Main Menu