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Spark Plugs

Spark plugs are one of the most misunderstood components of an engine. Numerous questions have surfaced over the years, leaving many people confused. Spark plug reading is a science all it's own, but there are some rules of thumb. Spark plugs are the "window" into your engine (your only eyewitness to the combustion chamber), and can be used as a valuable diagnostic tool. Like a patient's thermometer, the spark plug displays symptoms and conditions of the engine's performance. The experienced tuner can analyze these symptoms to track down the root cause of many problems, or to determine air/fuel ratios.


The spark plug has two primary functions: To ignite the air/fuel mixture and to remove heat from the combustion chamber.

Spark plugs transmit electrical energy that turns fuel into working energy. A sufficient amount of voltage must be supplied by the ignition system to cause it to spark across the spark plug's gap. This is called "Electrical Performance." The temperature of the spark plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called "Thermal Performance", and is determined by the heat range selected. It is important to remember that spark plugs do not create heat, they can only remove heat. The spark plug works as a heat exchanger by pulling unwanted thermal energy away from the combustion chamber, and transferring the heat to the engine's cooling system. The heat range is defined as a plug's ability to dissipate heat.

Light gray or tan deposits and slight electrode erosion

Over Heating:
An extremely white insulator with small black deposits and premature electrode erosion.
Symptoms: Loss of power at high-speed or during heavy load
Causes: Plug insufficiently tightened.
Engine insufficiently cooled.
Ignition timing too advanced.
Plug heat range too hot.

Carbon Fouling:
Dry, soft black carbon on the insulator and electrodes.
Symptoms: Poor starting, Misfiring, and Faulty acceleration
Causes: Faulty choke - over rich air/fuel mixture
Delayed ignition timing
Bad ignition leads
Plug heat range too cold

Oil Fouling:
Wet, oily black deposits on the insulator and electrodes.
Symptoms: Poor starting and Misfiring.
Causes: Wrong piston rings, cylinders, and valve guides.
Fuel mixture oil content too high (two-stroke engines).

A melted or burned center and/or ground electrode, blistered insulator and aluminum or other metallic deposits on the insulator.
Symptoms: Loss of power causing engine damage. Pre-ignition occurs when combustion begins before the timed spark occurs.
Causes: Plug insufficiently tightened.
Engine insufficiently cooled.
Ignition timing too advanced.
Plug heat range too hot.