How To Read Spark Plug Color

From: AJS Tech Staff
Date: 1/7/00

Comments

Reading spark plugs can be a valuable tuning aid. By examining the spark plug insulator color, an experienced personal watercraft engine tuner can determine valuable information about the engine's overall operating condition.

Normal: Grey to Light Golden-Brown Color

• This condition is ideal, the spark plug and engine air/fuel mixture are operating properly.

Dry: Black Soot Buildup

• Air/fuel mixture is too rich, the carburetor settings are incorrect, or the flame arrestor is dirty or has mounting problems.
• Spark plug heat range is too cold for the operating conditions.
• Ignition system problems causing a weak or intermittent spark.

Wet Fouling: Shiny, Wet, Black Appearance

• Excessive use of the choke (gas fouled)
• Prolonged low rpm operation (gas or oil fouled)
• Fuel to oil ratio is too rich (oil fouled)

Excess Deposits: Bumpy, Chalky Buildup

• Poor fuel quality
• Oil leakage into combustion chamber
• Improper oil used for premix/injected

Overheated: White, Blistered, Melted Electrode

• Lean air/fuel mixture
• Spark plug heat range is too hot for operating condition of the engine
• Plug is not properly gapped and/or torqued onto head
• Overly advanced timing

A detonation problem would show signs, such as silver specs, black specs, or melting or breakage at the firing tip. See article on Pre-Ignition & Detonation Causes for more information.

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

Last changed: September 08, 2008