Gas Walk-Behind Mower: Won't start

When you can't start your mower, make sure the tank has fuel and that the fuel is fresh; stale fuel can become watery and won't ignite well. Tune up the mower's engine to give it the best chance of starting. Check the spark plug and replace it if it's covered with oil or other deposits.

If your mower uses a battery, charge or replace the battery if it won't power the starter to spin the engine. If you have to pull the starter rope to start the engine, replace the recoil starter if it doesn't spin the engine when you pull the rope. Replace the starter rope if it's broken. For additional help, watch our can't pull the recoil starter rope video.

Replace the safety switch or the ignition coil if the spark plug isn't getting current through the ignition system. The engine won't start if you hit a rock or stump and shear off the flywheel key, so replace the flywheel key if it's broken. Our spark plug and ignition problems video provides additional troubleshooting tips when the engine isn't getting any spark.

Carburetor problems often prevent a mower engine from starting. Replace or rebuild the carburetor if it's clogged. Our fuel, ignition and compression video has more help troubleshooting fuel problems.

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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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
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Replace a lawn mower ignition coil

Replace a lawn mower ignition coil

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.

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

The mower ignition coil could be the problem if your mower won't start when you pull the starter rope. This easy-to-follow repair guide and video explain how to replace the lawn mower ignition coil yourself.

Repair difficulty
Time required
 30 minutes or less
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How to replace a lawn mower ignition coil on a flathead engine

When your lawn mower won't start because there's no spark, the problem might be the mower ignition coil—especially if the spark plug is fine. This DIY repair guide shows how to replace the ignition coil on an older mower model that has a flathead engine.

Repair difficulty
Time required
 30 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
Read more
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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Replace a lawn mower recoil starter assembly

Replace a lawn mower recoil starter assembly

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.

How to replace a lawn mower recoil starter on a flathead engine

When your lawn mower recoil starter rope won't retract after you pull it, the problem could be a broken recoil spring. It's easiest to replace the assembly rather than just the spring. This lawn mower repair guide shows how to replace the recoil starter in 30 minutes or less. This guide is for older lawn mowers models that have a flathead engine.

Repair difficulty
Time required
 30 minutes or less
Read more
How to replace a lawn mower recoil starter on an OHV engine

Can't start your lawn mower because the starter rope locked up? A broken recoil starter spring or pulley could be to blame. This step-by-step repair guide and video show how to replace a lawn mower recoil starter on a newer model with an OHV engine.

Repair difficulty
Time required
 30 minutes or less
Read more
Replace a lawn mower safety switch

Replace a lawn mower safety switch

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.

April 1, 2013
By Lyle Weischwill
How to replace a lawn mower safety switch

If your lawn mower won't start, or if it won't stop when you release the control bar, the problem could be the safety switch. This DIY repair guide explains how to replace a lawn mower safety switch in about half an hour.

Repair difficulty
Time required
 30 minutes or less
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Charge or replace a lawn mower battery

Charge or replace a lawn mower battery

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.

Replace a lawn mower starter rope

Replace a lawn mower starter rope

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.

April 1, 2013
By Lyle Weischwill
How to replace a lawn mower starter rope

Lawn mower starter rope broke when you pulled it? This easy-to-follow repair guide shows how to replace the lawn mower starter rope in 30 minutes or less.

Repair difficulty
Time required
 30 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.