Gas Walk-Behind 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.

gas walk-behind mower

These repairs may help solve your gas walk-behind mower problem

Replace a lawn mower clogged carburetor

Replace a lawn mower clogged 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.

March 1, 2016
By Lyle Weischwill
How to replace a lawn mower carburetor

When your lawn mower runs rough or won't start at all, the carburetor might be clogged or damaged. This DIY repair guide and video show how to replace a lawn mower carburetor in 30 minutes or less.

Repair difficulty
Time required
 30 minutes or less
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Rebuild a lawn mower carburetor

Rebuild a lawn mower carburetor

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.

March 1, 2016
By Lyle Weischwill
How to rebuild a lawn mower carburetor

If your lawn mower won't start or runs rough, the carburetor could be damaged or clogged. Follow the steps in this DIY repair guide and video to rebuild a lawn mower carburetor in about half an hour.

Repair difficulty
Time required
 60 minutes or less
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Replace a lawn mower spark plug

Replace a lawn mower 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.

March 1, 2016
By Lyle Weischwill
How to replace a lawn mower spark plug

If your lawn mower engine runs rough or is hard to start, a new spark plug could be the right fix. This repair guide and video explain how to replace a lawn mower spark plug.

Repair difficulty
Time required
 15 minutes or less
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Replace a lawn mower flywheel key

Replace a lawn mower 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.

How to replace a lawn mower flywheel key on an OHV engine

If your lawn mower died after hitting a rock or tree stump, the flywheel key might have broken. This repair guide and video show how to replace a lawn mower flywheel key.

Repair difficulty
Time required
 60 minutes or less
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How to replace a lawn mower flywheel key on a flathead engine

If your lawn mower doesn't start after hitting a rock or a stump, the mower flywheel pin might have sheared to protect expensive components. This repair guide shows how to replace a lawn mower flywheel key. This guide is for older mower models that have a flathead engine.

Repair difficulty
Time required
 45 minutes or less
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Tune up a lawn mower

Tune up a lawn mower

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.

March 1, 2016
By Lyle Weischwill
How to replace a lawn mower spark plug

If your lawn mower engine runs rough or is hard to start, a new spark plug could be the right fix. This repair guide and video explain how to replace a lawn mower spark plug.

Repair difficulty
Time required
 15 minutes or less
Read more
Symptoms for gas walk-behind mowers

Choose a symptom to see related walk-behind mower repairs.

Main causes: drive control cable failure, worn or broken drive belt, bad transmission, broken drive wheel
Main causes: damaged cutting blade, loose cutting blade, damaged flywheel key, engine needs tune up
Main causes: dirty carburetor, bad spark plug, clogged air filter, engine choke problems, clogged gas cap vent
Main causes: stale gas, engine needs tune up, bad spark plug, dead battery, bad recoil starter, faulty safety switch, bad ignition coil
Main causes: engine needs tune up, dirty or clogged carburetor, damaged flywheel key
Main causes: uneven wheel height settings, damaged wheel, dull or damaged cutting blade
Repair guides for gas walk-behind mowers

These step-by-step repair guides will help you safely fix what’s broken on your walk-behind lawn mower.

How to replace a lawn mower ignition coil on an OHV engine

Replace the ignition coil if it isn't sending electric current to the spark plug when you pull the recoil starter rope.

Repair difficulty
Time required
 30 minutes or less
How to replace a lawn mower flywheel key on an OHV engine

The flywheel key couples the flywheel to the engine crankshaft. Replace the flywheel key if it's broken.

Repair difficulty
Time required
 60 minutes or less
March 1, 2016
By Lyle Weischwill
How to replace a lawn mower drive cable

The drive cable engages the transmission to spin the drive wheels and move the mower across the grass. Replace the drive cable if it's broken or frozen up inside the sheath.

Repair difficulty
Time required
 60 minutes or less
Articles and videos for gas walk-behind mowers

Use the advice and tips in these articles and videos to get the most out of your walk-behind lawn mower.

May 31, 2019
How to tune up a lawn mower video
By Sears PartsDirect staff

See how to tune up your mower once a year to keep it running well.

February 1, 2016
6 weeks to a better lawn
By Erin Hynes

Do these 5 things this spring for a lawn that's healthier all summer.

April 1, 2015
How to keep grass clippings from sticking to a mower deck
By Sears PartsDirect staff

Prevent your mower deck from rusting by following our advice for keeping grass clippings from sticking.