Why the Spark Plug Won't Fire on a Lawn Mower

When the spark plug doesn't fire or fires intermittently, your lawn mower engine won't start, or if it does start, it will stall. The plug is an electrical device that gets its power from an induction coil connected to the engine flywheel and ignites the fuel in the combustion chamber. If you suspect that the one on your lawn mower isn't firing, the fault could be with the fuel, the induction coil or the plug itself.

How the Spark Plug Operates

The spark plug screws into a hole on the engine in such a way so that its electrode protrudes into the combustion chamber while its other end remains outside, available for connection to the spark plug wire. The wire is connected to an induction coil that produces a voltage whenever the coil is in the correct orientation to a fixed magnet situated on the rim. The voltage produces an electric current that flows to the plug, and when it reaches the plug electrodes, it jumps across the gap between them and makes a spark, much like lightning jumps between a cloud and the ground.

Worn and Dirty Plugs

A spark plug won't fire properly if its electrodes have been worn thin, if they are covered with combustion deposits or if they're wet. If the plug has seen many hours of service, the electrodes may just be worn out, and replacing the plug will restore normal engine operation. If other parts of the engine are malfunctioning, however, you can often glean information about the malfunctions by examining the plug. Excessive dry deposits or wearing of the electrodes usually points to a problem with the fuel-to-air ratio in the combustion chamber, and oily deposits may indicate a broken engine seal.

Reading a Spark Plug

Removing your spark plug from the engine is a simple job, but it requires a socket wrench deep enough to hold the plug. A plug in normal condition has a thin layer of light-brown deposits on it. If you see a thick layer of black carbon deposits, it's a sign that the carburetor mixture is too rich -- your engine may even be smoking. Electrodes that have been worn thin are a sign of overheating, often caused by too much ethanol in the gas. If your plug has wet, oily deposits, it's time to take the mower in for servicing, because oil is leaking into the combustion chamber where it shouldn't be.

Other Engine Problems

If your spark plug appears to have normal wear -- or if you've just installed a new one -- and the engine doesn't fire, you can check the plug by replacing the spark plug wire with a tester and pulling the starting rope. If you don't see a spark on the tester, there may be a problem with the ignition coil of the flywheel to which it's attached. The ignition coil doesn't frequently develop problems, but the connection between the flywheel and coil or between the starting pulley and flywheel can wear out, and the coil may not be turning. Replacing the worn part isn't difficult, but you may need a pro to properly disassemble the engine.

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