Bad Gas in a Lawn Mower | Signs & Fixes

Gasoline doesn’t last forever. After about a month sitting in a fuel tank, untreated gas starts breaking down. It oxidizes, forms a gummy varnish, and clogs the tiny passages in a lawn mower’s carburetor. If the mower also smells like old paint thinner when you pull the cord, bad gas is almost certainly the cause. The fix involves draining the whole system, cleaning what’s clogged, and starting fresh with stabilized fuel.

How To Tell If The Gas In Your Mower Is Bad

Bad gas shows three clear signs that don’t require special tools to spot. First, the fuel itself looks wrong — cloudy instead of clear, or darker than fresh gasoline. Second, it smells sour or varnish-like, nothing like the sharp odor of new pump gas. Third, if you pour a little onto absorbent paper and it leaves a sticky residue, the fuel has oxidized into gum. Any one of these means the gas has degraded and needs to come out.

The Complete Drain Sequence

Topping off a tank of bad gas with fresh fuel rarely works — the old fuel still clogs the carburetor. Draining the entire fuel system is required. Start with the fuel tank, then move to the carburetor bowl.

Step 1: Drain The Fuel Tank

Disconnect the fuel line from the carburetor where it connects. Siphon or pour all old fuel from the tank into an approved container. A fluid extraction pump makes this cleaner, but a turkey baster works in a pinch. When the tank is empty, dispose of the old gas at a local hazardous-waste collection center — never pour it on the ground or into a drain.

Step 2: Drain The Carburetor Float Bowl

Locate the small drain plug on the underside of the carburetor’s float bowl. On most Briggs & Stratton and Honda engines, a 10mm or 13mm socket loosens it. Before you start, tilt the mower onto a paver stone under the handle — this keeps fuel from flowing out the tube and into the bowl while you work. Let the fuel drain completely, then tighten the plug back down.

Step 3: Clean The Carburetor And Jets

Remove the air filter. Spray carburetor cleaner directly into the intake opening and let it sit several minutes to dissolve varnish inside. Then remove the float bowl cover and spray the float and the narrow fuel passages. For a clogged main jet, bend a wire tie into an L-shape and insert it into the jet tube — wiggle it to dislodge any debris. If the engine still runs rough after reassembly — what mechanics call a “three-legged dog” — spray AMSOIL Power Foam into the carburetor to clear remaining deposits.

Before buying a replacement mower, try this fix sequence — it solves the vast majority of bad-gas problems. If you’re already shopping for a new machine, our roundup of tested budget gas mowers covers the best options that start reliably out of the box.

Spark Plug And Air Filter Checks After A Bad-Gas Incident

Bad gas doesn’t always damage the spark plug, but the plug is worth checking while the carburetor is open. A properly running four-stroke engine spark plug lasts for years and should never look oily or burned. If the plug shows either, replace it and set the gap to the specification in your owner’s manual — typically 0.025 to 0.030 inches for most Briggs & Stratton engines. Use a spark plug tester or clip the boot to the plug and hold it against the cylinder head while pulling the starter cord — a strong blue spark is what you need. The air filter also needs attention: foam filters wash clean with soap and water, while rigid filters get tapped out and cleaned with compressed air blown from the inside out, never from the outside in.

The Ethanol Connection And Why It Makes Bad Gas Worse

Most US gasoline contains up to 10% ethanol (E10). Ethanol absorbs moisture from the air, and over a month or more that water separates from the fuel — a process called phase separation. The water sinks to the bottom of the tank and the carburetor bowl, where it can rust the bowl from the inside and prevent the engine from running. Non-ethanol fuel avoids this problem entirely and is the best choice for any small engine used seasonally. VP Racing Fuels and several other brands sell ethanol-free gas in pre-packaged cans or at dedicated pumps. If ethanol fuel is all you can get, using a stabilizer every time you fill up helps slow the degradation.

Preventing Bad Gas From Coming Back

The fix only lasts if you keep the next tank of gas from going bad the same way. Add a fuel stabilizer to every fill-up, not just the end-of-season tank — fuel can degrade in 30 days even during mowing season. Run the mower for a few minutes after treating the gas so the stabilized fuel reaches the carburetor. If you park the mower for more than a month, either drain the tank and carburetor bowl completely, or fill the tank with stabilized non-ethanol fuel and run it long enough to circulate. This one habit prevents 90% of bad-gas problems over the life of the mower.