Lawn mower engines are remarkably similar to the gasoline-powered internal combustion engines used in automobiles, just on a much smaller scale. While automobile engines are equipped with several spark plugs, lawn mowers are equipped with only one spark plug because there is only one cylinder. Despite this, the principles used to diagnose spark plug performance in automobile engines can be used to diagnose on lawn mowers, too. Spark plugs rarely stop working, but a spark plug in poor condition is often indicative of some other problem with the engine.
Pull the spark plug wire off the tip of the spark plug.
Clean away any dirt and debris surrounding the base of the spark plug to prevent any containments inadvertently falling into the cylinder when the spark plug is removed.
Remove the spark plug by turning it in a counterclockwise direction with a deep-set socket and a socket wrench.
Examine the tip of the spark plug, where the plug enters the engine, for signs of wear. A grayish-colored tip indicates that the engine is in working order. Replace the plug at the interval recommended by the manufacturer. The piston rings are likely damaged if the tip of the spark plug is covered with oil. If the tip is covered with black ash, the engine's air/fuel mixture is probably too rich or the choke may be sticking. If the porcelain tip of the spark plug is broken, the air/fuel mixture is probably too lean or the engine oil level may be low.
Check and adjust the spark plug gap if necessary. The distance between the two metal points on the tip of a spark plug is referred to as the "spark plug gap." Measure the spark plug gap with a spark plug gauge and compare that measurement to the measurement called for in the lawn mower's owner's manual. Adjust the distance by carefully bending the hook-shaped point.
Tighten the spark plug into the engine until snug and press the spark plug wire onto the spark plug.