Gas Walk-Behind Mower: Why won't my lawn mower start?

A clogged orifice or jet in a carburetor prevents the engine from getting the mix of gas and air it needs. When the carburetor is clogged, the engine won't start, runs rough or won't stay running. It's usually more effective and cost efficient to replace the carburetor rather than rebuild it.

A clogged or dirty carburetor causes the engine to run rough or prevents the engine from starting. If you can start the engine using starter fluid but the engine won't continue to run, clogged carburetor jets may be the problem. Clean and rebuild the carburetor using a carburetor rebuild kit if the engine won't start or it runs rough because the carburetor isn't supplying the engine with fuel.

Tune up a walk-behind mower when the engine runs rough or won't start. You may need to tune up the engine when it vibrates too much or won't stay running. Regular tune-ups prevent engine wear and improve engine efficiency. A walk-behind mower tune-up includes changing the oil, spark plug and air filter. It could also include sharpening or replacing the blade. Follow the steps in our How to tune up a lawn mower video to get your mower running smoothly again.

The ignition coil sits next to the flywheel and generates electrical current to fire the spark plug when magnets on the flywheel pass the ignition coil. If the ignition coil fails, the engine won't start because the spark plug doesn't fire. Connect a spark plug tester to the engine to see if the spark plug is getting electrical current from the ignition coil. If the ignition coil doesn't work, replace it.

The spark plug gets electrical current from the ignition coils and sparks to ignite the fuel mixture in the cylinder, which drives the piston. The spark plug won't spark if it's coated with carbon deposits or oil residue, or if there's a crack in the ceramic insulator. The engine won't start or runs rough if the spark plug is bad. Replace the spark plug if it's damaged or coated with residue.

The flywheel key is a small metal rectangle that keeps the crankshaft and flywheel aligned when you tighten the flywheel nut. To protect more expensive components from damage, the flywheel key is designed to shear if the mower blade hits an object like a rock or tree stump hard enough to make the flywheel slip out of alignment with the crankshaft. If the flywheel key gets dented when the blade hits an object, the engine runs rough or vibrates severely. If the flywheel key shears when you hit an object, your lawn mower will stop running and won't start. If you inspect the flywheel key and find it dented or sheared, replace it.

The recoil starter on top of the mower spins the spins the flywheel to start the engine. The mower won't start with a broken recoil starter, so replace the recoil starter if it won't spin the flywheel. The recoil starter has a spring that retracts the starter rope used to pull-start a gas mower. If the rope doesn't retract after you pull it, it's likely the spring is broken. Replacing the spring itself is difficult and dangerous, so the best way to repair a recoil starter is to replace the whole recoil starter assembly.

The engine safety switch on a walk-behind lawn mower stops the ignition coil from sparking and has a brake pad that slows the flywheel. If the safety switch fails, the engine might not turn off (in that case, remove the spark plug wire to kill the engine). A failed safety switch also prevents the engine from starting because the switch prevents the ignition coil from sparking the spark plug. If the coil isn't sparking, remove the kill wire from the ignition coil and check the engine for spark. If there's spark, replace the safety switch.

If your lawn mower won't start using the electric starter, the starter might not be receiving power from the battery. Batteries fail over time or if improperly maintained. If charging the battery overnight doesn't correct the problem, replace the battery.

The starter rope is a heavy nylon cord that coils around the pulley and spins the recoil starter when you pull the rope. When the rope breaks, you can't spin the recoil starter to start the engine. Replace the starter rope if it's broken.