Should you drain the gas before storing a lawn mower for winter?

Yes, you should either drain the fuel or add a stabiliser before a lawn mower sits through winter, because petrol left untreated for months is the leading cause of hard starting in spring. As petrol ages, the ethanol in it attracts moisture and the fuel forms gums and varnish that clog the tiny passages in the carburettor.

Left in a mower over a cold, damp winter, that stale fuel turns the carburettor into the single most likely thing to fail. Spending a few minutes on the fuel before storage is far easier than stripping and cleaning a gummed carburettor in spring, which is why every engine maker builds a storage step into its manual.

Should you drain the tank or stabilise the fuel?

Both work well, so choose whichever suits you: drain the tank for the cleanest storage, or stabilise a full tank if you would rather not handle loose fuel. The important thing is that no untreated petrol is left to stale over the months of layup.

The two approaches, step by step, are below.

  1. To drain: siphon the petrol into an approved container, then start the engine and run it until it stalls to burn off what remains in the carburettor.
  2. To stabilise: fill the tank with fresh petrol, add a fuel stabiliser at the dose on the bottle, and run the engine for a few minutes so the treated fuel reaches the carburettor.

A full, stabilised tank has a small extra advantage, because it leaves less air space for condensation to form than a half-empty one. A drained mower, by contrast, is lighter and holds no fuel at all, which some owners prefer for a machine stored in an attached garage or shed.

What else should you do while the mower is stored?

Storage is the natural time to complete the rest of the winter service, since the mower is already out and idle. Change the engine oil, clean or replace the air filter, remove and inspect the spark plug, and clear caked grass from under the deck.

The end-of-season jobs worth doing together are listed below.

  • Change the engine oil, so the engine does not sit all winter on used, acidic oil.
  • Clean or replace the air filter, ready for an easy start in spring.
  • Remove, inspect, and re-gap or replace the spark plug.
  • Scrape and rinse clippings from under the deck to slow rust.
  • Sharpen or replace the blade so it is ready for the first cut.

Disconnect the spark plug wire before you tip the mower or work near the blade, and store the machine somewhere dry and away from any appliance with a pilot flame.

What happens if you skip this step?

Skip the fuel step and the most likely result is a mower that will not start in spring, because the stale petrol has varnished inside the carburettor. That usually means removing and cleaning the carburettor, or paying a shop to do it, before the mower runs again.

A worked example shows the payoff. A mower left full of untreated E10 through a damp winter commonly needs its carburettor stripped and cleaned come spring, a job of an hour or more or a service bill. The same mower drained, or filled with stabilised fuel and run for a few minutes beforehand, typically starts within a pull or two. The few minutes spent on the fuel in autumn save that whole headache.

How long can stabilised fuel sit?

Stabilised petrol typically stays usable for several months to about a year, depending on the product and storage, which comfortably covers a normal winter layup. The stabiliser slows the oxidation and moisture uptake that would otherwise gum the carburettor.

For the treatment to work you must add the stabiliser to fresh fuel, not fuel that has already begun to stale, and run the engine for a few minutes so the treated petrol reaches the carburettor rather than sitting only in the tank. Stored somewhere cool and dry, a stabilised, full tank then gives the mower its best chance of an easy first start in spring.

Final safety note

Handle all of this outdoors, because petrol vapour is flammable and heavier than air, so it pools and travels to find a spark. Never drain or store fuel near a boiler, water heater, or any pilot flame.

Our lawn mower maintenance guide covers the full seasonal routine, and our gas lawn mower guide helps if you are choosing your next mower. Petrol is flammable, so drain and store it outdoors away from any spark, keep an extinguisher handy, and follow your mower's manual.