Almost every mower that will not start in April was put away wet in October. Learning how to winterize a lawn mower is really learning one thing: what stale fuel does to a carburettor over five idle months. Everything else on the list is worth doing, but the fuel is the part that decides whether spring starts with a pull cord or a repair bill.
Quick answer: To winterize a lawn mower, deal with the fuel first: either run the tank dry, or fill it and add fuel stabilizer then run the engine 5 to 10 minutes so treated fuel reaches the carburettor. Change the oil while the engine is still warm, clean the deck, sharpen the blade, clean or replace the air filter, and fog the cylinder through the spark plug hole. Store it dry and off a damp floor. For a battery mower, skip all the fuel steps and store the pack indoors at around half charge.
Key takeaways
- Fuel is the whole job. Stale, water contaminated petrol in a carburettor is the number one cause of spring no starts.
- Pick one fuel strategy and commit. Run it dry, or fill it and stabilize it. Half a tank of untreated fuel is the worst option.
- Stabilizer must be run through the engine. Pouring it in the tank and switching off leaves the carburettor full of untreated fuel.
- Change the oil warm, in autumn, not spring. Used oil carries acids and moisture that sit in the engine all winter.
- Sharpen the blade now. The job is identical in October and you get an April where you just mow.
- Battery mowers need the opposite. No fuel steps, but the pack matters: about half charge, indoors, never freezing.
Why fuel is the thing that kills mowers
Modern pump petrol is usually E10, meaning 10 percent ethanol, and that is the limit for small engines like mowers. The problem is what ethanol does while sitting still.
Penn State Extension puts it plainly: gasoline is not water soluble, but ethanol is, so ethanol can pick up contaminants that gasoline does not and deposit them inside your engine, leading to fouled filters and injectors. They also note that engines which are rarely or seasonally used, such as lawnmowers and chainsaws, are especially prone to these difficulties, and that on older equipment ethanol can make seals and hoses deteriorate, shrink, or swell, causing leaks.
Their storage advice is the actionable part: add fuel stabilizer if the equipment will be idle for more than a few weeks, and avoid keeping stored fuel for long periods. Some manufacturers go further and recommend stabilizer mixed with the fuel at all times, not just for storage.
What that looks like in a carburettor after five months: the light fractions evaporate, leaving a varnish that blocks passages thinner than a hair, and any water the ethanol absorbed sits in the float bowl and corrodes it. That is the mower that “just stopped working over winter”.
Run it dry, or fill and stabilize?
Both work. Doing neither is what causes problems. Pick based on your machine and how long it sits.
| Run the tank dry | Fill and stabilize | |
|---|---|---|
| Best for | Small push mowers, short storage | Riding mowers, metal tanks, long storage |
| How | Run until it stalls, restart until it will not fire | Fill to about 95 percent, add stabilizer, run 5 to 10 min |
| Upside | No fuel left to go stale | No air space for condensation, seals stay wet |
| Downside | Rubber seals can dry and shrink | Stabilizer only buys months, not years |
The one rule that applies to both: a part full tank of untreated fuel is the worst case. The air space above the fuel lets moisture condense, and the ethanol pulls it straight in.
And if you stabilize, you must run the engine afterward. Stabilizer in the tank does nothing for the carburettor, which is where the varnish forms. Five to ten minutes of running pulls treated fuel through the fuel line, the jets, and the float bowl, which is the entire point.
How do you winterize a lawn mower step by step?
- Do the fuel step first, with the engine running. Either run it dry, or add stabilizer to a full tank and run it 5 to 10 minutes. Do this before everything else, because the next step wants a warm engine anyway.
- Change the oil while it is still warm. Warm oil carries the suspended dirt out with it, and used oil holds combustion acids and moisture that will sit against bearings all winter. Doing it now rather than in spring is one of the few genuinely free wins in mower maintenance. The procedure is in how to change lawn mower oil.
- Disconnect the spark plug lead. Before your hands go anywhere near the blade. Every time.
- Clean the deck properly. Scrape the packed clippings off the underside. Wet grass against steel is a rust factory over five damp months, and the caked layer holds moisture like a sponge.
- Sharpen the blade now. Same job in October as in April, except in April you want to be mowing. Follow how to sharpen lawn mower blades, and remember why it matters: a dull blade tears grass, as covered in how to mow a lawn.
- Clean or replace the air filter. Paper filters get replaced, foam ones washed and lightly oiled. A clogged filter is the second most common spring no start after fuel.
- Fog the cylinder. Pull the spark plug, put a teaspoon of clean engine oil into the hole, pull the cord slowly two or three times to spread it over the bore, then refit the plug. This coats the cylinder wall so it does not surface rust while the engine sits.
- Fit a new spark plug, or clean and gap the old one. Plugs are cheap and it is far easier now than kneeling on cold concrete in spring.
- Deal with the battery, if it has one. Electric start mowers: take the battery off, charge it fully, store it somewhere above freezing, and top it up mid winter. A flat battery left in the cold is a dead battery.
- Store it dry and off the floor. A pallet or a couple of battens keeps it off damp concrete. Cover it with something breathable, not a plastic sheet, which traps condensation against the metal. If it lives in a shed, read how to stop condensation in a shed, because a sweating shed will rust a perfectly winterized mower.
Battery mowers are a different job
Cordless mowers skip every fuel step, which is most of this article, and that is a real advantage. But the battery has its own rules and getting them wrong costs more than a carburettor clean.
- Store at partial charge, not full and not empty. Somewhere around half is the usual guidance. A lithium pack left at 100 percent for five months ages faster; left at zero it can drop below the point where the charger will touch it again.
- Bring it inside. Room temperature, dry. Not a freezing shed, not a baking loft.
- Take it off the mower so nothing draws on it slowly all winter.
- Check it mid winter and bring it back up if it has drifted down.
- Everything else still applies: clean the deck, sharpen the blade, and store the machine dry and off the floor.
If your petrol mower has reached the stage where it needs a carburettor rebuild every spring, that is worth weighing against a replacement. The best electric cordless lawn mowers remove this entire ritual, and the best value cordless lawn mowers cover the budget end. If you would rather stay with petrol, the best gasoline lawn mowers and the best lawn mower brands are the place to start.
Common winterizing mistakes
- Leaving a part full tank of untreated fuel. The single most damaging thing you can do. Air space plus ethanol equals water in the float bowl.
- Adding stabilizer and switching off. The carburettor never sees it. Run the engine.
- Leaving the oil until spring. Acids and moisture sit in the crankcase for five months.
- Storing it with a grass packed deck. Damp clippings hold water against bare steel all winter.
- Covering it with a plastic sheet. Condensation forms under it and you have made a small humid tent around your engine.
- Leaving a battery in a freezing shed. Often the most expensive part of the machine, killed for free.
- Storing fuel from last year and using it next spring. Extension guidance is explicit about avoiding long fuel storage. Buy fresh.
- Skipping the spark plug lead. Every deck job starts with disconnecting it.
If you did none of this and spring has already arrived, the fix is usually the carburettor rather than the engine, and how to fix a lawn mower that will not start walks through the diagnosis in order.
Frequently asked questions
Should you drain the gas from a lawn mower for winter?
Either drain it completely or fill the tank and add stabilizer, then run the engine 5 to 10 minutes so treated fuel reaches the carburettor. Both work. Leaving a part full tank of untreated fuel is the worst option, because the air space lets moisture condense and ethanol absorbs it.
Do you need fuel stabilizer for a lawn mower?
Yes, if fuel will sit in the machine for more than a few weeks. Extension guidance recommends adding stabilizer for equipment that will be idle that long, and avoiding long term storage of fuel generally. Some manufacturers suggest using stabilizer in all fuel, not just before storage.
Should you change mower oil before or after winter?
Before, in autumn, while the engine is still warm from running. Warm oil flows out carrying suspended dirt, and used oil contains combustion acids and moisture that would otherwise sit against internal parts for months. Changing it in spring leaves that contamination in place all winter.
How do you store a battery lawn mower for winter?
Remove the battery, leave it at roughly half charge rather than full or empty, and store it indoors at room temperature. Check it mid winter and top it up if it has drained. Clean the deck and sharpen the blade as usual, and keep the mower off a damp floor.
Why will my mower not start in spring?
Usually stale fuel. Over months the light fractions evaporate and leave varnish blocking carburettor passages, while ethanol absorbed water sits in the float bowl. The other common causes are a clogged air filter and a fouled spark plug. All three are prevented by winterizing rather than fixed cheaply afterward.
Is it bad to store a lawn mower in an unheated shed?
The cold is not the problem; the damp is. An unventilated shed condenses moisture onto cold metal overnight, which rusts a deck and corrodes contacts. Ventilate the shed, keep the mower off the floor on battens, and cover it with something breathable rather than plastic sheeting.
Final thoughts
Winterizing a lawn mower is an hour in October that buys you a working machine in April. Deal with the fuel by running it dry or stabilizing a full tank and running it through, change the oil warm, clean the deck, sharpen the blade, fog the cylinder, and store it dry and off the floor. If you are on batteries, the fuel half disappears and the pack becomes the thing to look after. And if the yearly carburettor ritual has worn thin, the best electric cordless lawn mowers make this whole article about four steps shorter.