Walk-Behind Lawn Mower: Not running well

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 or engine.

See the repair guides below and watch our troubleshooting video for instructions on correcting common causes.

Troubleshooting video: Lawn Mower Won’t Start: Fuel, Ignition and Compression Problems.

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 sputters and dies when you try to start it. 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. 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 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 shear slightly, the engine runs rough or misfires. If the flywheel key breaks or shears severely, your lawn mower stops running and won’t start. If the flywheel key breaks or shears, replace it.

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

Change a lawn mower oil and air filter

Oil

You should change your lawn mower's oil every 50 hours and before storing the machine in the fall. Over time, heat and friction break down the oil's ability to lubricate moving parts; changing the oil can help extend engine life. Change or clean the air filter, depending on your model; as it becomes clogged, it works less efficiently.

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