If your lawn mower’s been sitting a while, the fuel supply might be the reason it won’t start. Yes, gas can go bad, and when it does, it gets sticky, loses power, and clogs fuel lines and other parts. You don’t need to be an engine mechanic to fix this. With a few simple steps, you can clear out the old fuel in question and get your mower running again. This guide will walk you through what to look for, what to do, and how to keep it from happening again.

Does Lawn Mower Gas Go Bad?

Yes, gas can go bad, and it doesn’t take long. After about 30 days, regular gasoline starts to break down. Over time, it loses its ability to combust properly and can cause all sorts of issues in engine-based lawn equipment like lawn mowers. If your mower smells like varnish or runs rough, there’s a good chance the fuel has gone stale.

When gas breaks down, it can:

• Leave behind sticky deposits and damaging residue that clog the carburetor and gum up the fuel tank
• Make starting the lawnmower engine harder or even impossible
• Cause engine damage, rough idling, and reduced overall performance

If your mower ran fine last season but won’t start this spring, the fuel tank is one of the first things to check.

How to Get Old Gas Out of a Lawn Mower

If you suspect the gas is old, the first step is to remove it. Don’t just top off the lawn mower gas tank—mixing old gas with new doesn’t fix the problem. Here’s how to get rid of the old fuel safely:

1. Work in a well-ventilated area on a flat surface.
2. Disconnect the spark plug wire for safety.
3. Use a siphon pump or turkey baster to pull the gas out of the gas tank and into a suitable container. Try to remove as much residual fuel as possible.
4. Drain the carburetor bowl/float bowl if your mower has one. This helps remove any leftover bad fuel sitting in the engine.
5. Dispose of old gas properly. Don’t pour stale fuel down the drain or on the ground—most towns have a waste facility that takes it.

Once the gas tank is empty, you can add fresh fuel. If your mower has a fuel filter, now’s a good time to check or replace it.

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Bad Gas in Lawn Mower: How to Fix It

If your mower still won’t start after changing the gas, old fuel might have gummed up the carburetor or other parts. Here's what you can try:

• Add a fuel system cleaner to the new gas. Some additives help break down deposits and clean things out of gas lines.
• Clean the carburetor if the mower still sputters or stalls.  The stale fuel residues may have clogged a jet hole or the float needle. This might mean removing it, but sometimes spraying carburetor cleaner into the air intake can help remove built-up residue in gummed-up carburetors.
• Replace the spark plug if it’s fouled. Bad gas can leave carbon deposits that affect firing.

If nothing works, a full carburetor cleaning or carburetor rebuild might be needed, but most of the time, fresh gas and a quick spray of cleaner will clear the gunk from carburetors and get things back on track.

FAQ

How long does it take for gas to go bad in a lawn mower?

Gas can start to go bad in as little as 30 days, especially if it's stored in a hot place or exposed to air. Using a fuel stabilizer as and extra measure before putting your mower into storage can help keep the fuel fresh longer, sometimes up to six months or more.

Can I use bad gas if I mix it with new gas?

It’s not a good idea. Old gas has lost its power and can still leave behind gummy residue that clogs parts. Mixing it with fresh gas may slightly improve things, but you’re still running the risk of future issues. It’s better to get rid of the old stuff completely and start with a fresh tank.