What Causes a Lawn Mower to Stop Running After Starting?

An internal combustion engine, like those found on a lawn mower, requires three basic things to start and to keep running. Air mixes with gas, fuel moves the cylinder and a spark ignites it, keeping the system moving through to the next cycle. While this process happens quickly on an engine, if you find the missing link you can get your mower running again.

If your lawn mower stops, check for fuel, spark and air.

Loss of Gas

Fuel flow must be unrestricted, so from the time it leaves the tank until it is ignited in the cylinder, the fuel must pass uninhibited. A loss in the supply of fuel to the cylinder will instantly stop the combustion cycle and shut down your engine. A loss of fuel can be caused by a clogged fuel cap, a dirty fuel tank, a clogged fuel filter, dirty fuel hoses, holes or leaks in the system, and dirty passages in the carburetor. Clean the entire fuel system and check for damaged or broken parts.

Loss of Spark

When the fuel reaches the top of the cylinder, the spark plug and ignition system ignite the fuel to further power the crankshaft. A loss of spark or a mistimed spark will cause the combustion cycle to stop immediately. Common lawn mower sparking problems include a dirty or fouled plug, loose or poorly connected boot and plug wires, faulty ignition switches, dirty flywheel magnets, a damaged flywheel, and a worn-out ignition module. Shut the ignition off, disconnect the spark plug and ground the plug wire before working on the ignition system.

Loss of Air

Air must continue to mix with the gas, and it also cools the engine. In addition, the engine needs to push the heated gases away from the internal parts of the engine. If these air passages get clogged, the air supply will get choked off, effectively shutting down the combustion process. Common air problems on a lawn mower include dirty or old air filters, a dirty carburetor choke, and a dirty muffler and spark arrestor screen. Wash the filter pad after every 10 hours of operation. Clean the muffler after every 60 hours of operation.

Cooling and Lubrication

If the engine can't vent the heat away from the piston and crankcase, the engine will shut down. Also, if the piston and crankcase heat up due to a loss of lubrication, the seals may wear out and shut the entire engine down. Always check the oil tank for sufficient lubrication before each use. Wipe off the air intake vents around the top engine cylinder cover. Keep dust and other debris from clogging up the fan housing and the engine will stay cool enough.