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Stabilize the fuel, run the carburetor dry, change the oil, disconnect the battery, and start it under load for 20 minutes once a month. That routine takes about half an hour up front and 20 minutes a month afterward, and it prevents almost everything that stops a stored generator from starting. The overwhelming cause of a generator that will not run after sitting is not a worn engine — it is ethanol-blended gasoline that degraded in the carburetor while the machine did nothing.
The fuel problem, which is most of the problem
Gasoline sold across the United States is typically E10, containing up to 10% ethanol — a consequence of the federal Renewable Fuel Standard described by the EPA, with background in the EIA's biofuels explainer. Ethanol is hygroscopic: it pulls water vapor out of the air through the tank vent. Once enough water accumulates the mixture phase-separates, leaving a layer of water and ethanol at the bottom of the tank and a fuel above it that has lost octane.
Untreated, this process begins within about 30 days and is well advanced by three months. Two things follow. Water and ethanol in contact with steel and aluminum corrode both. And as the light fractions evaporate out of the carburetor float bowl, what remains is a sticky varnish that dries across the jets.
Those jets are the crux. A small-engine carburetor meters idle and low-speed fuel through passages measured in thousandths of an inch. It takes a trivial amount of varnish to block one completely, at which point the engine cranks strongly, has spark and compression, and will not fire. The owner concludes the generator is dead. It needs a carburetor clean — $90 to $200 at a small-engine shop, and the reason those shops are booked solid every hurricane season.
Prevention is a stabilizer added at fill time and a carburetor emptied before storage. That is it.
Best for every tank and every gas can that will sit longer than a month
- Treats up to 40 gallons per 16 oz bottle
- Keeps fuel usable for up to 24 months
- Formulated against ethanol-related corrosion and gum
- Add at fill time, not after the fuel has aged
Why buy it: stale ethanol-blended gasoline is the overwhelming cause of a generator that will not start after sitting, and eight dollars of stabilizer prevents most of what people pay a shop to fix.
Best for machines that already sat with old fuel, and seasonal small engines generally
- Enzyme formula intended to disperse water and break down gum
- Small-engine dosing on the bottle
- Usable as both a stabilizer and a treatment for aged fuel
- Works with ethanol-blended gasoline
Why buy it: the small-engine formula is dosed for the tank sizes generators actually have, which is where owners most often get the concentration wrong.
Best for keeping dust, rodents and condensation off a stored inverter generator
- Sized for 2,000 to 4,000-watt inverter generators
- Weather-resistant fabric with an elasticized hem
- Keeps dust and debris out of vents and the control panel
- For storage, not for use while the generator runs
Why buy it: a storage cover is not a running cover — this one keeps a stored machine clean, and running under it would trap exhaust and heat.
The storage routine, step by step
- Fill the tank and add stabilizer at the correct dose. Full is better than partly empty, because an air space breathes moist air in and out with temperature swings. Dose by the bottle's instructions for the tank volume — over-treating is not better.
- Run the engine for 10 minutes so treated fuel reaches the carburetor. Stabilizer that never leaves the tank protects the tank and nothing else.
- Close the fuel shut-off valve and let it run until it stops. This empties the float bowl, which is where varnish forms. If your machine has no shut-off valve, drain the bowl using its drain screw, or drain the tank and run it dry. This step matters more than any other.
- Change the oil while the engine is still warm. Warm oil carries suspended contaminants out with it. Old oil is acidic and will pit bearing surfaces over a long layup. Replace the filter if the engine has one.
- Deal with the spark plug. For storage over six months, remove the plug, put a teaspoon of clean engine oil into the cylinder, pull the recoil slowly a couple of times to spread it on the bore, and refit the plug. This prevents cylinder-wall rust in humid storage.
- Disconnect the starting battery on an electric-start machine and keep it on a maintainer, or at minimum charge it every three months. A lead-acid battery left discharged over a winter sulfates and will not recover.
- Clean the machine. Wipe off oil and fuel residue, clear debris from the cooling fins and the air intake, and check the air filter.
- Block the exhaust and intake loosely — a rag or a purpose-made plug — against mice, wasps and dirt daubers. Write yourself a note, because a forgotten rag in an exhaust is its own problem.
- Store it dry, off the floor, under a breathable cover. Concrete wicks moisture; a pallet or a shelf stops it. Avoid sealed plastic sheeting, which traps condensation against the metal.
Full tank or empty tank?
| Storage period | Tank | Carburetor | Reasoning |
|---|---|---|---|
| Under 30 days | Full, treated | Leave it | Fuel is still good; condensation is the only risk |
| 1–6 months | Full, treated | Run dry | Stabilizer holds the tank; the bowl is the vulnerable part |
| 6–12 months | Full, treated, or drained | Run dry | Either works if the fuel is treated at fill |
| Over 12 months | Drain completely | Run dry and drain the bowl | No stabilizer is indefinite; remove the fuel |
| Seasonal, exercised monthly | Full, treated | Refreshed by the monthly run | The best case — fuel never sits long enough to degrade |
Note the trap in the "drain it" advice: draining the tank alone is not enough, and is arguably worse than leaving treated fuel in. The tank empties, the bowl does not, and the small volume left in the bowl evaporates completely and leaves the heaviest varnish deposit of all. If you drain, drain the bowl too.
The monthly exercise run
This is the single most valuable habit in generator ownership, and the one most owners skip.
Run the machine under a real load — a couple of space heaters, a shop vacuum, work lights, anything that puts a genuine several hundred watts through it — for about 20 minutes, once a month. A 30-second no-load run is worse than nothing: it introduces fuel and condensation into a cold engine and never reaches the temperature that drives moisture back out.
Twenty minutes under load does four things. It pulls fresh, treated fuel through the whole fuel path. It brings the crankcase hot enough to evaporate accumulated water. It charges the starting battery. And most importantly, it tells you the generator works — in daylight, in fair weather, when a fouled plug or a flat battery is a fifteen-minute errand instead of a crisis.
If you own a dual fuel machine, running the monthly exercise on propane keeps the engine exercised while leaving no varnish behind at all. That is a genuine practical argument for dual fuel that has nothing to do with fuel cost — see best dual fuel generators.
Bringing a stored generator back
- Check the oil level and condition before anything else. Top up or change if it looks dark or smells of fuel.
- Reconnect and check the battery, or charge it if it has been sitting.
- Remove any exhaust or intake plugs. Inspect for nests — mice and wasps are the second most common cause of a stored machine failing.
- Fill with fresh gasoline. If old fuel is in the tank, drain it and dispose of it at a household hazardous-waste facility; do not pour it into a car.
- Open the fuel valve, wait a minute for the bowl to fill, and start.
- Run at no load for five minutes, then apply load progressively and confirm output at the receptacles.
- If it cranks but will not fire, the odds heavily favor a blocked carburetor jet rather than an engine fault. That is a cleaning job, not a replacement.
Five storage mistakes
Leaving untreated fuel in it. The defining mistake of the category, and the reason small-engine shops are full every September.
Draining the tank but not the bowl. The bowl is where the varnish forms. Half the job leaves the worse half undone.
Brief no-load starts instead of a real exercise run. A cold 30-second run adds moisture and removes none.
A sealed plastic cover. Trapping condensation against metal for six months causes exactly the corrosion you were trying to prevent. Use a breathable cover.
Storing it on a concrete floor. Concrete wicks ground moisture into whatever sits on it. A pallet or a low shelf costs nothing and solves it.
Where to go next
If the machine is new, start with how to break in a new generator. For fuel planning and consumption figures, see how much gas a generator uses. The propane route that avoids most of this is in best dual fuel generators, and machine selection in best generators for home backup. Everything sits under the power tools and workshop hub.
We never buy or physically handle the equipment we write about — this guidance is drawn from manufacturer service documentation and aggregated owner experience, and your model's manual overrides any general figure here. The method is on how we rank. Product links are affiliate links; they earn us a small commission at no cost to you and never change a recommendation.