How to Winterize a Lawn Mower

1. The One-Hour Checklist

If you only do one thing, do the fuel. If you do two, add the oil. The rest is the difference between a mower that lasts eight years and one that lasts fifteen.

2. Fuel: The Only Step That Really Matters

Modern pump gasoline is not a stable product. It begins losing its light, volatile fractions within about thirty days, and the ethanol blended into most of it is hygroscopic — it pulls water straight out of the air through the tank vent. Given enough time and a temperature swing, that water and ethanol separate out and settle as a corrosive layer at the bottom of the tank and in the carburettor float bowl.

What remains behind as the light fractions evaporate is a sticky varnish that dries into the tiny fuel passages of a carburettor. Main jets on a walk-behind mower are drilled to a diameter smaller than a pin, and it takes almost nothing to block one.

Option A: Run It Dry

Use up what is in the tank on the last mow, then let the engine run until it stalls from fuel starvation. That last part is not optional. Emptying the tank while leaving the carburettor bowl full accomplishes very little, because the bowl is exactly where the varnish forms. If your mower has a fuel shut-off valve, close it and let the engine run out.

Running dry suits machines that will sit five or six months and owners who would rather not buy stabilizer. The one downside is that a completely dry carburettor can dry out rubber seals and diaphragms over a long storage, which matters more on older machines than new ones.

Option B: Fill and Stabilize

Add stabilizer at the dose on the bottle, fill the tank to about ninety percent, and then run the engine for five full minutes. Those five minutes are the step everyone skips, and without them the treated fuel never reaches the carburettor — the bowl stays full of the untreated fuel that was already in it.

A full tank leaves little air space for moisture, and stabilizer extends usable fuel life to roughly a year. If you can buy ethanol-free fuel, it is a genuine upgrade for any machine that sits between uses, and the premium is small relative to a carburettor rebuild.

3. Oil, and Why Autumn Beats Spring

Used engine oil is not simply dirty. It carries acidic combustion by-products and condensed water, and a small air-cooled engine that runs in short bursts never gets hot enough for long enough to boil that moisture off. Leaving it in the sump for five stationary months gives those acids an uninterrupted run at bearing surfaces and cylinder walls.

Drain it while the engine is warm from its last run, because warm oil flows and carries suspended contamination out with it rather than leaving it settled in the corners. Refill to the mark with the grade your manual specifies — usually SAE 30 or 10W-30 for a walk-behind — and check the level on flat ground rather than on the slope where you happened to park.

Doing this in autumn rather than spring also means the machine is genuinely ready when the grass starts growing. The spring version of this job invariably happens on the first warm Saturday, when it competes with actually mowing the lawn.

4. Deck, Blade and Underside

Packed grass on the underside of a deck is a wet sponge held permanently against steel. Over one winter it produces surface rust; over several it produces the thin, perforated deck edge that eventually ends a mower's life. Scrape it off with a putty knife or a plastic scraper — a stiff wire brush finishes the job — and get into the corners around the discharge chute where it packs hardest.

With the deck clean and the plug lead disconnected, inspect the blade. Look for nicks deeper than the sharpening will remove, cracks radiating from the centre hole, and a trailing edge worn thin and rounded. A cracked blade is scrap, not a sharpening candidate: it is spinning at around 3,000 rpm and a failure is genuinely dangerous.

Sharpen at the original bevel angle, take equal material from both ends, and check balance on a nail through the centre hole before refitting. An unbalanced blade vibrates, and that vibration goes straight into the crankshaft bearings. Our guide to how often to sharpen mower blades covers the technique and the sharpness standard in detail.

A light coat of oil or silicone spray on the clean deck underside slows rust and makes next season's clippings release more easily. It takes a minute and is worth doing.

5. Plug, Filter and Cylinder

A spark plug is a few dollars and replacing it annually removes an entire category of spring starting problems. If you would rather clean and reuse, wire-brush the electrode, check the gap against the manual figure, and look at the insulator colour: light tan is correct, black and sooty means it has been running rich, and white and blistered means it has been running hot.

Air filters matter more than their price suggests. A paper element gets tapped clean or replaced; a foam element gets washed in warm soapy water, dried completely, and lightly re-oiled if the manual specifies oiling. Never fit a damp foam filter — you will draw water into the carburettor, which is the opposite of the goal here.

6. Battery and Electric Mowers

There is no fuel to worry about, which removes the main failure mode entirely — but lithium packs have their own storage rules, and getting them wrong costs capacity that never comes back.

Store the pack at roughly 40 to 60 percent charge, not full and not empty. A lithium cell held at full charge ages measurably faster, and one left to self-discharge to zero over five months can drop below the voltage its protection circuit will accept, at which point the charger simply refuses to acknowledge it. Most modern packs have a charge-level indicator; use it.

Take the pack indoors. Room temperature is ideal, a heated basement is fine, and a garage floor at −10°C is not. Charging a cold lithium pack is what actually damages it, so if a pack has been stored cold, let it reach room temperature before putting it on the charger.

Everything else is the same as a gas machine: clean the deck, sharpen and balance the blade, check the wheels and height adjusters, and store it dry. Corded electric mowers need only the deck and blade work, plus a cable inspection along its full length for nicks and crushed sections.

If your mower's battery is one of the older or heavier packs and it is starting to fade, note the capacity now while you remember, and read our battery mower testing before spring rather than during it.

7. Where to Store It

An unheated garage or shed is fine. Cold is not the enemy — cycling between cold and warm is, because that is what drives condensation onto cold metal. A stable cold space is better than a space that swings twenty degrees every time the sun comes out.

Get the mower off bare concrete if you can, on a pallet or a sheet of plywood. Concrete slabs wick moisture upward continuously, and a machine parked directly on one sits in a permanently humid microclimate.

Cover it with something breathable — a cotton sheet or a proper vented cover. A plastic tarp seals moisture in and produces exactly the rust you were trying to prevent. If mice are a possibility, and in a shed they always are, block the exhaust and air intake with steel wool, and write yourself a note to remove it. A nest in an exhaust silencer is a memorable spring discovery.

Finally, keep fuel and mowers apart from anything with a pilot light. Petrol vapour is heavier than air and travels along the floor, and a water heater at the back of the garage is an ignition source at exactly the wrong height.

8. Waking It Up in Spring

If you stabilized a full tank, the fuel is usable and you simply check the oil level, refit the battery or reconnect the plug lead, and start it. Expect a little smoke in the first thirty seconds if you fogged the cylinder.

If you ran it dry, add fresh fuel, then give the fuel pump or gravity feed a moment to fill the bowl before cranking repeatedly. Repeated pulling on an empty carburettor floods nothing and achieves nothing except an aching shoulder.

If it will not start and you did winterize it, work in order: fuel present and fresh, plug lead firmly on, plug sparking, air filter clear, and the safety bail lever properly engaged. That last one is responsible for a surprising share of no-start calls after a machine has sat and the operator has forgotten the routine.