Ethanol Gas and Small Engines: What It Really Does

A mower that ran fine in July won’t start in April. A generator that sat in the garage since the last ice storm cranks and cranks and never catches. A trimmer runs for thirty seconds, surges, and dies. Different machines, same story — and nine times out of ten the cause isn’t the engine. It’s what was left sitting in the tank.

Almost every gallon of regular pump gas in Kentucky is E10: up to ten percent ethanol. Your truck doesn’t care, because it burns through a tank in a week. Small engines are a different situation entirely, and the reason comes down to time.

What Ethanol Actually Does

Two properties cause nearly all the trouble.

1. Ethanol pulls water out of the air

Ethanol is hygroscopic — it attracts and holds moisture. A vented fuel tank breathes with every temperature swing, and Kentucky gives it plenty to work with: warm afternoons, cool nights, humid summers. Every cycle pulls a little damp air in, and the ethanol in the fuel grabs the moisture out of it.

For a while, the water stays mixed in and nothing seems wrong. Then it crosses a threshold and the fuel separates into layers. The ethanol and water drop to the bottom of the tank as a heavier, corrosive layer, and the gasoline floats on top. That’s phase separation, and it’s the point of no return — you cannot shake it back together.

The cruel part is where the fuel pickup sits: at the bottom. So the engine draws the water layer first. That’s the machine that cranks strong and won’t fire, and it’s why the gas can look perfectly normal from the fill cap.

2. Ethanol is a solvent

Ethanol is harder on rubber and certain plastics than straight gasoline. In an engine built in the last decade or so, the fuel lines, gaskets, and carburetor components are made to handle it. In older equipment — and small engines stay in service a long time — it softens fuel lines, swells or shrinks gaskets, and breaks down primer bulbs.

It also acts like a cleaner inside a tank that’s been running non-ethanol fuel for years, loosening old varnish and scale and sending it straight into the carburetor. That’s a real thing we see: someone switches fuel and the engine gets worse before it gets better.

Why Small Engines Suffer and Cars Don’t

It’s not that small engines are fragile. It’s that they sit.

• Fuel turnover. A car cycles its whole tank weekly. A mower might use two gallons across a season; a generator can go a year untouched.
• Vented tanks. Automotive fuel systems are sealed and pressurized. Most small engine tanks breathe openly with the weather — exactly the condition that lets moisture in.
• Tiny passages. A carburetor’s main jet has openings smaller than a pin. Deposits that a fuel-injected car would never notice will block one completely.
• Storage conditions. Unheated garages, sheds, and barns swing forty degrees in a day. Every swing is another breath of damp air.

What Bad Fuel Looks Like

You can usually spot it before you ever pull the carburetor:

• Cranks but won’t start after sitting through a season — the classic.
• Starts, runs 20–60 seconds, dies. It’s burning what’s in the bowl and finding nothing behind it.
• Surging or hunting — the engine speed rising and falling on its own at a steady throttle.
• Only runs with the choke on. A partly blocked jet leans the mixture out; the choke masks it.
• A sharp, sour, varnish-like smell from the tank instead of a normal gasoline smell.
• Cloudy fuel, or a visible line between two layers in a clear filter or a drained sample. That line is phase separation.
• Gummy amber or white crust inside the carburetor bowl — what’s left after the fuel evaporated.
• Soft, sticky, or cracked fuel lines and a primer bulb that’s gone hard or mushy.

E10, E15, and Ethanol-Free

Three things worth knowing at the pump:

• E10 (up to 10% ethanol) is standard regular pump gas. Modern small engines are built to run on it. Fresh, it’s fine. Stale, it’s the problem described above.
• E15 (up to 15% ethanol) is labeled at the pump and is not approved for small engines — mowers, ATVs, generators, chainsaws, trimmers, boats. Manufacturers exclude it, and it can void your warranty. Read the pump label, especially when fuel prices push stations toward higher-ethanol blends.
• Ethanol-free (E0) is sold at some stations and marinas, often at a premium and sometimes only in a higher octane. For anything that sits between uses, it’s the single easiest thing you can do to avoid a repair bill.

Our honest advice: if the machine gets used weekly through the season, E10 with reasonable habits is fine. If it sits — generators, chainsaws, backup equipment, anything seasonal — run ethanol-free. The price difference on the couple of gallons those machines burn in a year is less than one carburetor job.

Fuel Stabilizer: What It Does and Doesn’t Do

Stabilizer slows the chemical breakdown of gasoline and helps limit the moisture problem. Used correctly, it works well. Used incorrectly, it does nothing, and that’s where most people go wrong.

• Add it to fresh fuel. Stabilizer is a preservative, not a repair. Poured into fuel that’s already gone bad, it cannot undo phase separation or dissolve varnish.
• Run the engine after adding it. Five to ten minutes, so treated fuel actually reaches the carburetor. Stabilizer in the tank does nothing for the fuel already sitting in the bowl and lines.
• Follow the mix ratio. More is not better; the ratio is on the bottle.
• Treat the can, not just the machine. The gas can in the corner of the garage is where a lot of bad fuel starts.

The 30-Day Rule

The habit that prevents most of these repairs is simple: untreated pump gas gets 30 days. Past that, either it’s been stabilized or it shouldn’t go in the engine.

Practically, that means buying fuel in amounts you’ll actually use. A five-gallon can is convenient in May and a liability by August. Buy two gallons at a time, use it, buy fresh. Old fuel in the can is a slow leak in your equipment budget.

Storing Equipment: Run It Dry or Fill It Up

There are two defensible approaches, and the wrong move is stopping halfway between them.

Run it dry

Shut off the fuel valve, if it has one, and let the engine run until it stalls, then drain the carburetor bowl. Nothing left to go bad, nothing left to gum up. Best for trimmers, blowers, chainsaws, and anything with a small tank.

Fill it up — with treated fuel

Fill the tank nearly full with stabilized fuel and run it long enough to pull treated gas through the whole system. A full tank leaves less airspace, which means less moisture pulled in. Better for larger tanks on riding mowers and zero turns, where running dry isn’t practical.

What you don’t want is the middle: half a tank of untreated gas sitting all winter, breathing damp air over an air gap. That’s the setup that produces the April no-start.

Either way, the rest of the storage routine still applies — and if you want the full sequence, see our guide to winterizing your ATV or mower.

What We Do When Bad Fuel Wins

Sometimes the fuel has already done its work. When one of these comes into the shop in Richmond, the job usually runs like this: drain the old fuel and dispose of it properly, pull and clean the carburetor — or rebuild it, if the jets and seats are far gone — replace the fuel filter and any lines or bulbs that have gone soft, clean out the tank if there’s residue in the bottom, then refill with fresh fuel and confirm it runs right under load, not just at idle.

Most of these are straightforward jobs, and many of them are same-day. Turf & Trail doesn’t do free estimates — you get a firm quote before any work begins, so you know the number before we touch it.

Ethanol & Small Engine FAQ

Is ethanol gas bad for small engines?
E10 runs fine in most modern small engines as long as it’s fresh. The damage comes from ethanol sitting still — absorbing water and attacking older rubber and plastic. Fuel that sits is the problem, not fuel that gets used.

How long can gas sit in a mower before it goes bad?
Treat 30 days as the limit for untreated pump gas. Stabilizer mixed into fresh fuel reasonably stretches that through a storage season. Stabilizer added to fuel that’s already gone bad won’t fix it.

Should I use ethanol-free gas?
On anything that sits between uses — generators, chainsaws, trimmers, seasonal equipment — yes. It costs more per gallon and it’s the cheapest insurance available.

Can you fix an engine that’s had bad gas in it?
Usually. Most bad-fuel jobs come down to draining the system and cleaning or rebuilding the carburetor, sometimes with new lines and a filter. Bring it in and you’ll get a firm quote before any work starts.