Solved! My Lawn Mower Won’t Stay Running. What’s Wrong With It?

Q: I can’t seem to figure out why my lawn mower won’t stay running. There’s plenty of gas and oil in it. Can it be repaired, or is it a lost cause?

A: Lawn mowers seem like simple machines, yet they require some TLC to operate smoothly—especially if they’ve been sitting in the shed for a while. A full gas tank and oil reservoir are the essential first steps when checking why the lawn mower won’t stay running, but the problem could also be a dirty filter, clogged carburetor, improper fuel mixture, or a dirty spark plug. If you’re ready to get your hands dirty, check out these troubleshooting tips to get your lawn mower running again. However, you may need a lawn mower specialist to get you back to trimming your grass. Alternatively, a landscaping professional can take care of it for you.

The fuel might be old and dirty and needs to be replaced.

Fuel that has sat in the motor for a while can evaporate and leave a sticky residue. The inlet and outlet ports leading to the carburetor could be clogged, as well as the fuel filter. In cold, wet winter seasons, condensation can collect inside the tank, which doesn’t mix with the fuel. Replace old, dirty, or diluted fuel; adding fresh fuel to it won’t solve the problem. Drain the old fuel out of the tank and collect it to be disposed of safely per local requirements. Clean off the carburetor ports before refilling the tank.

The air filter might be dirty, which prevents it from getting enough air.

Motors have air filters to collect and trap dust and debris that could clog the machine’s inner workings. Over time, they become clogged and need to be replaced. If an air filter isn’t working correctly, the motor can’t get enough air to prevent overheating. Inspect the air filter for carbon deposits or oil, and replace it if it’s contaminated. Paper filters always need immediate replacement if they’re contaminated, but a dusty foam filter could benefit from being washed and dried at least once before replacement. It’s recommended to replace the air filter after every 25 hours of use.

The carburetor might be clogged and dirty.

One inconvenient reason a lawn mower won’t stay running is a problem with the carburetor, which mixes the air and fuel for internal combustion. The carburetor bowl may be clogged, or the inlet and outlet ports are slimy from fuel residue. Remove them and spray them with an aerosolized carburetor cleaner (not water). You can try disconnecting the fuel lines and spraying them out with cleaner to remove any sitting sludge as well. Replacing the fuel filter is also recommended if the carburetor is showing signs of contamination. If the lawn mower has a gas cap vent, open it to clear any visible clogs. If none of these steps solve the problem, it’s worth having the carburetor replaced by a pro.

You forgot to reset the choke, so the fuel mixture is too rich.

Most lawn mowers have a choke to modify the fuel-to-air mixture, which keeps the engine running until it’s warmed up. Unless you’ve had cool weather for a while, the choke should be off to prevent overloading the motor with the wrong mixture of fuel. If the choke is left on for too long, excess fuel flows into the combustion chamber, causing it to seize up and stall. If this happens, the lawn mower will need to sit idle for several hours to allow the fuel to drain out of the chamber. Turn off the choke and try again later. If the problem still isn’t solved, a faulty choke switch could be the problem. Clean it with carburetor spray or replace it entirely.

A dirty spark plug is misfiring and needs to be replaced.

A faulty or dirty spark plug could be the reason your lawn mower won’t stay running. It might produce the initial spark, but the spark won’t be enough to keep the engine running if the plug is clogged with dirt and oil. Spark plugs are usually located on the front of a push lawn mower and connected to a black cable. Remove it with a socket wrench and clean with a wire brush and some brake cleaner. A dark, sticky carbon residue indicates a need to replace the spark plug. Especially if you have repeated times when the lawn mower won’t stay running, scheduling regular tune-ups can help keep spark plugs in great shape all year.