How To Repair A Lawn Mower Engine Misfire

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Spark Plug

The spark plug ignites the fuel in the combustion chamber once it has been compressed. If the spark plug is no longer firing, or corroded with residue and cannot produce a strong enough current to properly spark the fuel, the engine may misfire or completely stop running altogether. If your engine is misfiring, stop the engine and let it cool for 20-30 minutes before removing the spark plug to check for wear or corrosion. Corroded spark plugs can often be cleaned. It may be convenient to have an extra spark plug handy so you can try replacing the plug altogether to be sure if a faulty spark plug is causing the engine to misfire. If you try a new spark plug and continue to have the same issue with your engine, the problem is likely coming from a different component.


Your lawn mower's engine timing may be off, causing it to misfire. The engine ignites just before top dead center to create maximum power. This is called “spark advance” This maximizes the energy created when the compressed fuel mixture is ignited. Engine timing refers to the relationship between the position of the crankshaft (and piston) and the camshaft (and valves). If the timing is off the valves will be open for a moment during the compression and combustion strokes. This will lead to poor or rough running, backfiring, and possibly loss of power. The timing can become off a number of ways. Most often it is a result of use and wear as the tight fit of all the mechanical components wear and loosen up. Many small engines have timing belts that can slip or break. Discontinue use of the mower immediately and bring it to a professional to have it serviced.


Every engine has two valves for each piston it possesses. One of the valves controls the flow of fuel and air into the cylinder, and the other allows the gasses leftover after combustion to leave the cylinder through the exhaust system. If the valve timing is off on your mowers engine, it may not run at all, or it will be running very roughly and possibly backfiring. Malfunctioning valves can also cause the engine to misfire, as the timing of the combustion process is thrown off which can cause the engine to spark at irregular and incorrect intervals.

Ignition Coil

The ignition coil is responsible for providing the spark plug with the current that allows it to spark and ignite fuel in the engines combustion chamber. This is usually accomplished with magnets on the flywheel spinning over inductors to generate current for the ignition coil to send to the spark plug. If the timing of the flywheel and magnets is off, the spark plug will not spark at the exact right time, and the engine will not be running at its full capacity. Ignition coils can fail and not work at all or be going bad and work somewhat intermittently. Setting the proper air gap between the coil and the flywheel when replacing a coil or doing other work on the engine that involves removing the coil. If you believe your ignition coil is failing, a simple spark test can be performed. The spark tester is hooked up inline with the spark plug boot (which transfers current from the ignition coil), and the spark plug. The spark tester will indicate whether the ignition coil is sending any current to the spark plug. If the spark plug is not receiving current, the ignition coil is broken and will need to be replaced.

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