How to Tell if Gas Is Bad in Your Lawn Mower (and What to Do)
In This Guide
Most mower problems we diagnose at driveways are not really mower problems, they are fuel problems wearing a disguise. Gasoline is a perishable product with a shorter shelf life than most owners assume, and the carburetor pays the price for every month it sits. This guide shows you how to check whether the gas in your tank has gone bad, what to do with it, the honest answer on mixing old and new fuel, and which gas to buy so the question stops coming up.
How Fast Gas Goes Bad (and Why)
Pump gasoline begins measurably degrading in roughly 30 days, and three things happen as it sits. The light, volatile compounds that make cold starting easy evaporate first, leaving fuel that ignites reluctantly. The remaining heavier compounds oxidize into gums and varnish, the sticky residue that clogs carburetor jets. And ethanol blends absorb water from humid air, eventually separating into a corrosive water and alcohol layer at the bottom of the tank, exactly where the fuel pickup drinks. Six month old gas is unreliable, and gas that overwintered untreated is the single most common cause of spring starting trouble we see.
How to Tell if the Gas Is Bad: The Three Checks
You can verify fuel condition in two minutes without tools:
- Smell it. Fresh gasoline has a sharp, almost sweet solvent bite. Stale gas smells sour, muted and varnish like, closer to old paint thinner. The nose test alone is right most of the time.
- Look at it. Drain a sample into a clear glass jar. Fresh fuel is pale straw to nearly clear; degraded fuel runs noticeably darker, toward amber or muddy brown. Let the jar sit a minute and look for a distinct layer at the bottom (water and ethanol separation) or floating particles and rust.
- Date it. If you cannot remember buying the gas, it is old. Fuel past 30 days untreated is suspect, past 90 is a problem, and fuel from last season is a carburetor cleaning waiting to happen.
What Bad Gas Does to Your Mower
Stale fuel produces a recognizable family of symptoms because they all stem from poor combustion and restricted jets: hard or no starting, an engine that starts then dies, the rhythmic rev of surging, rough running, reduced power under load and increased smoke. Left long enough, the gums harden inside the carburetor and the symptoms stop being about the fuel and start being about the varnish it left behind, which is when a tank swap alone no longer fixes it and the carburetor needs physical cleaning, covered in our guide on the signs your carburetor needs cleaning.
Can You Mix Old Gas With New Gas?
The honest answer is a qualified yes. Old gas that is merely stale, not contaminated, can be blended into fresh fuel at a modest ratio, around one part old to four or more parts fresh, and most engines will burn it without complaint, especially a car, which tolerates marginal fuel far better than a small carbureted engine. The conditions: the old gas must pass the jar test with no water layer, no particles and no rust, and the blend belongs in a vehicle tank more than a mower tank.
Do not blend at all if the fuel shows separation, debris or a strong varnish odor, and never feed a mower carbon pure old gas to use it up. The few dollars of fuel you save buys a $115 carburetor cleaning. Truly bad gas goes to your county household hazardous waste site, not down a drain and not into the lawn.
What to Do With a Mower That Has Old Gas in It
If the mower sat with untreated fuel, work this order:
- Drain the tank. A siphon or turkey baster works on mowers without a drain plug. Capture the fuel for disposal or dilution.
- Drain the carburetor bowl. One bolt on the bottom of most float carburetors empties the cup where varnish concentrates.
- Refill with fresh fuel and a dose of stabilizer, then attempt a start. Many mowers come back to life right here.
- If it still struggles, the varnish has already done its work inside the jets. Our walkthroughs on a mower that cranks but will not start and finding the carburetor take you the rest of the way, or a technician cleans it at your driveway for a flat rate.
The Best Gas for Lawn Mowers
Small engines want three things from fuel:
| Choice | Recommendation | Why |
|---|---|---|
| Octane | 87 regular | Mower engines are low compression; premium buys nothing |
| Ethanol | E10 maximum, ethanol free ideal | Ethanol attracts water and attacks rubber fuel parts; never use E15 or E85 |
| Freshness | Buy monthly, stabilize every can | Stabilizer extends usable life from one month to a year or more |
Ethanol free fuel, sold at some stations and in cans as premixed small engine fuel, is the gold standard for equipment that sits between uses. For everyday mowing, fresh stabilized E10 from a busy station serves fine. The expensive habit is the big can filled in April that pours its last stale gallon into the mower in September.
Storing Fuel and Mowers the Right Way
Prevention is one habit: no untreated fuel ever sits more than a month, in the can or in the mower. Add stabilizer at every fill up, run the engine a few minutes after adding it so treated fuel reaches the carburetor, and before winter either run the system dry or store it full of stabilized fuel, never half full of August gas. Cans live sealed, out of sun and temperature swings. The complete off season routine, including oil, blade and battery care, is in our maintenance guide, and the spring side of the calendar is covered in the spring tune up guide.
Old Gas Already Did Its Damage?
A Wildwood technician can diagnose and fix it in your driveway, usually in about an hour. More than 10,000 repairs completed since 2019, with flat rates published before we arrive.