What Is Your Spark Plug Trying to Tell You?

Richard L. Parish  |  11/18/2004 9:27:05 PM

The engines on your lawn mower, string trimmer, leaf blower or garden tractor all require one or more spark plugs.
 
Those spark plugs need to be inspected annually and replaced as needed. Looking at the condition of the spark plug(s) after removal can tell you a great deal about the condition of your engine. If you carefully examine the plug(s), you can gain insights into other maintenance needs on your engine. You might be able to correct performance problems or avoid more major repairs or even engine failure.

Normal
A normal plug appearance in 2-stroke or 4-stroke engines is light tan to gray, and the electrodes will not be burned.

Cold Fouling, 4-Stroke
If a spark plug from a 4-stroke engine is covered with a dry, black carbon coating, it may be cold fouled. Causes include the wrong plug (too "cold"), excessive idling or light loads, carburetor choke out of adjustment, defective spark plug wire, carburetor adjusted too rich or low engine compression.

Wet Fouling, 4-Stroke
If a spark plug from a 4-stroke engine has a wet, black oily film over the entire end of the plug, it may be wet fouled. Causes include worn valve guides, worn oil rings, plugged breather or plugged breather valve in the tappet chamber.

Wet Fouling, 2-Stroke
The appearance will be the same as a wet-fouled 4-stroke plug. Causes include the wrong plug (too cold), excessive idling, improper fuel:oil mix ratio or carburetor adjustment set too rich.

Overheating, 4-Stroke
If a spark plug from a 4-stroke engine has a gray or white blistered appearance or burned electrodes, the engine may be overheating. Causes include plugged cooling fins, bent or damaged blower housing, missing shields, carburetor set too lean or spark plug improperly tightened.

Overheating, 2-Stroke
The appearance will be the same as an overheated 4-stroke plug. Causes include the wrong plug (too hot), carburetor set too lean, sticking piston rings, engine overloaded or cooling fins plugged.

Your spark plug is trying to tell you what your engine needs. Pay attention to what it is telling you!

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