Walk-Behind Lawn Mower: Runs rough

If your lawn mower cuts out, smokes, idles poorly, vibrates or runs erratically, it could need a good tune-up to replace the spark plug, oil and air filter. Or the problem could be with the lawn mower's fuel system. Rebuild or replace the carburetor if the engine isn’t getting a good supply of fuel.

If the engine misfires, backfires, sputters or jerks the starter rope out of your hand when you try to start it, the flywheel key could be broken. A broken flywheel key disrupts the ignition timing because the magnets aren't passing the ignition coil in synchronization with piston movement. Replace the flywheel key if it’s damaged or broken.

These repairs may help solve your Walk-Behind Lawn Mower problem:

Replace a lawn mower clogged carburetor

Carburetor

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.

For manuals, repair guides, and specific part recommendations, enter your model number.

Rebuild a lawn mower carburetor

Carburetor rebuild kit

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.

For manuals, repair guides, and specific part recommendations, enter your model number.

Replace a lawn mower spark plug

Spark plug

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.

For manuals, repair guides, and specific part recommendations, enter your model number.

Replace a lawn mower flywheel key

Flywheel key

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.

For manuals, repair guides, and specific part recommendations, enter your model number.

Tune up a lawn mower

Air filter

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.

For manuals, repair guides, and specific part recommendations, enter your model number.

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