How to get old gas out of a Small Engine (and When you don't Have to)

The mower sat all winter, and now the gas in the tank is months old. The first instinct is to dump it and start over. Sometimes that's the right move. Often it isn't. Whether old gas needs to come out depends on how far gone it is, and a tank that smells a little sour is a different problem from one holding a layer of water at the bottom. This guide, a hands-on companion to our complete small engine fuel guide, covers how to drain old gas out of a lawn mower or small engine safely, how to tell when the fuel is bad enough that draining is the only real option, when you can treat and reuse it instead, and how to get rid of whatever you pour out.

How do you get old gas out of a lawn mower?

Siphon the fuel out of the tank with a hand pump or fluid siphon, then empty the carburetor bowl so old gas isn't left downstream. Work outdoors on a cool engine, keep sparks and flames away, and catch everything in an approved gasoline container. Two places can hold fuel: the tank contains most of it, while the carburetor contains a much smaller amount that can contribute to hard starting and fuel-system deposits.

The tank is the easy part. A hand siphon can pull most of the fuel into your container, and a turkey baster or suitable fuel-safe tubing can help remove what the siphon can't reach. On mowers with an easily removable tank, removing it and carefully pouring the fuel into an approved container may be simpler.

The carburetor deserves attention because gasoline left there can oxidize and leave gum or varnish deposits that restrict the small passages and jets. Carburetor designs vary by engine. Some have a drain screw or bowl bolt that allows fuel to be drained into a suitable container. Others may require draining through the fuel line or following the manufacturer's procedure for emptying the fuel system. If you're unsure which method applies to your mower, check the owner's or engine manual before loosening carburetor components.

Here's the safe order of operations:

1. Let the engine cool completely, move the mower outdoors, and disconnect the spark plug wire so the engine can't accidentally start.
2. Siphon the tank into an approved, labeled gasoline container.
3. Drain the remaining fuel from the carburetor according to the engine manufacturer's procedure.
4. Wipe up any spills, make sure everything you disconnected or loosened is properly reinstalled, reconnect the spark plug wire, and refill with fresh fuel.

Do you have to drain old gas, or can you treat it instead?

You don't always have to drain older gasoline. Fuel that has aged somewhat but remains clear and uniform, has no visible water or phase separation, and has not developed a strong sour or varnish-like odor may still be usable. Diluting it with a larger amount of fresh gasoline and adding an appropriate gasoline treatment can be a practical option. But additives cannot turn severely degraded gasoline back into fresh fuel.

One of the clearest reasons to drain a tank is water contamination or phase separation. Ethanol-blended gasoline can absorb moisture, and when enough water accumulates, the ethanol and water can separate from the gasoline and settle into a lower layer. Bell Performance technical data shows that water contamination and phase separation can dramatically shorten the useful storage life of ethanol-blended gasoline. Once distinct layers have formed, simply adding a fuel treatment should not be expected to restore the fuel to its original condition. The separated fuel should be removed.

Age alone doesn't necessarily tell you that gasoline must be discarded. Untreated gasoline can begin oxidizing and losing desirable properties in as little as 30 days, according to Briggs & Stratton, but that doesn't mean gasoline suddenly becomes unusable on day 31. Storage conditions, temperature, exposure to air and moisture, ethanol content, and whether the fuel was stabilized all affect how quickly it deteriorates.

If the fuel still looks clear and uniform and has no obvious water contamination or strong varnish-like odor, diluting a modest amount with fresh gasoline and treating it may be reasonable. Our guide to what to do with old gas walks through that judgment call in more detail. A treatment such as Ethanol Defense or Mix-I-Go can help manage moisture and protect fuel that is still usable. What these products cannot do is reverse severe oxidation or restore gasoline that has already undergone full phase separation.

How can you tell if old gas is bad enough to drain?

Old gas should generally come out when it has a strong sour or varnish-like odor, looks unusually dark or cloudy, contains visible water or debris, or has separated into distinct layers. Gas that remains clear and uniform and still smells reasonably like gasoline may still be usable, particularly if it hasn't been stored for an unusually long period or under poor conditions.

Three quick checks can tell you a lot. Smell it carefully without deliberately inhaling the vapors: fresh gasoline has its familiar petroleum odor, while badly oxidized fuel can develop a distinctly sour or varnish-like smell. Look at it: a small sample in a suitable clear container can reveal cloudiness, sediment, water or distinct layers that aren't obvious when looking into the mower's tank. Finally, consider its history: gasoline that has been sitting untreated for months, particularly through large temperature swings or humid conditions, deserves more scrutiny than fuel that was stabilized and stored properly.

Engine behavior can provide another clue, although it can't diagnose bad fuel by itself. Hard starting, surging, rough running or an engine that starts and quickly stalls can result from stale or contaminated gasoline, but those symptoms can also come from carburetor deposits, restricted fuel flow, ignition problems or other mechanical issues. If you suspect the fuel, inspecting a sample is a better test than diagnosing its condition from engine behavior alone.

Bell publishes a fuel health checklist for situations like these, because identifying deteriorated or water-contaminated fuel before it reaches sensitive fuel-system components is much easier than dealing with deposits and corrosion afterward.

How do you dispose of old gasoline safely?

Take unusable gasoline to a household hazardous waste collection site or another facility that specifically accepts old fuel. Never pour gasoline on the ground, down a drain or into household trash. Many county or municipal waste programs operate permanent collection sites or scheduled drop-off events, and some recycling or automotive facilities may accept gasoline depending on local rules. Keep the fuel in a sealed, approved and clearly labeled container until you can take it to an appropriate facility.

There's a real hazard behind each rule. Gasoline poured onto soil can contaminate groundwater. Poured down a drain, its vapors can create serious fire and explosion hazards. Putting gasoline into household trash can expose sanitation workers and equipment to flammable vapors and spills. Local regulations vary, so check with your municipal or county household hazardous waste program for the correct disposal procedure where you live.

And remember that age alone doesn't necessarily make gasoline hazardous waste. If you're unsure whether fuel is merely aged or genuinely contaminated or degraded, evaluate its condition before deciding what to do with it. But once gasoline shows obvious water contamination, phase separation, heavy oxidation or other signs that it is no longer suitable for use, proper hazardous-waste collection is the safest choice.

How do you keep gas from going bad in the first place?

Buy gasoline in quantities you'll use within a reasonable period, keep stored fuel tightly sealed in an approved container, and minimize exposure to heat, moisture and large temperature swings. If a mower, generator or other small engine will sit for an extended period, treating fresh gasoline with an appropriate fuel stabilizer before storage can significantly reduce the problems associated with oxidation and fuel deterioration.

The best time to stabilize gasoline is while it's still fresh, rather than waiting until it has already deteriorated. Before storing seasonal equipment, add the treatment at the recommended rate and run the engine long enough for treated fuel to move through the fuel system and into the carburetor. That helps protect not just the gasoline in the tank but also the small amount of fuel sitting in the carburetor, where oxidation and deposits can create starting problems later.

Ethanol-free gasoline can also be a useful option for equipment that sits for extended periods because it avoids the moisture-related behavior associated with ethanol-blended gasoline. It isn't immune to aging, however. Ethanol-free gasoline can still oxidize and form deposits during long storage, so good storage practices and appropriate stabilization still matter.

Here's how Bell's two main gasoline treatments map to small-engine use.

Ethanol Defense carries professional-strength water absorbers and does its work without adding alcohol to the fuel, while Mix-I-Go stabilizes gasoline and helps control deposits associated with fuel deterioration. Used appropriately, both can help extend the useful storage life of gasoline and reduce the chances that you'll be draining a bad tank when it's time to use the equipment again.