How to winterise a petrol lawn mower — UK guide

The fuel problem in 90 seconds

UK petrol is E10 (10% ethanol). Ethanol absorbs water from the air and separates from the petrol at the bottom of the tank.

Separated water-ethanol attacks brass jets in the carburettor, leaving a varnish-like residue. By spring, the carb is clogged.

This is the cause of 95% of spring no-start problems. Every Briggs, Honda, Stiga, Kohler, Kawasaki, and Loncin carb suffers from it.

Solution: either remove the fuel before winter, or stabilise it so ethanol does not separate.

Option A: Drain the fuel (simplest)

Run the mower until it stops from fuel starvation. This empties the tank, fuel lines, and (importantly) the carburettor float bowl.

Engine will sit dry over winter. No varnish forms because there is no fuel.

Best for: owners who use the mower less than once a fortnight in season anyway, or owners with no fuel-stabiliser product to hand.

Option B: Stabilise the fuel (easier on storage)

Add a fuel stabiliser (£8 for a season's supply — Briggs Fresh Start, STA-BIL Storage, or any branded equivalent) to the tank.

Run the mower for 5 minutes after adding stabiliser to circulate it through the carb.

Top the tank fully, so condensation cannot form in air space above the fuel.

Best for: owners who like to start the mower briefly in February on the first dry weekend to confirm it runs.

1. Change the engine oil

Cold oil holds acids and water from combustion. Leaving it in the engine over winter pits cylinder walls.

Drain the oil while warm (run the mower for 5 minutes first), refill with fresh SAE 30 or 10W-30 to the dipstick mark.

Old oil goes in a sealed container to your council recycling centre — never down a drain.

2. Clean the deck thoroughly

Damp grass clippings on the underside of the deck rot through the winter. Always clean before storage.

Tilt the mower (spark plug disconnected, carb side up) and scrape clippings off with a paint scraper or stiff brush.

Touch up any chipped paint on the deck — £4 spray can of mower paint. Prevents winter-spring rust gaining hold.

3. Inspect the blade and tighten bolts

Winter is the right time to sharpen the blade (see our blade-sharpening guide). Saves the spring rush at dealers.

Check all visible bolts — engine mounts, deck-handle joins, wheel axles. Vibration loosens bolts over a full season; tightening now prevents drop-damage in spring use.

Lubricate the throttle cable, drive cable (on SP mowers), and any pivot points with PTFE spray or thin oil.

4. Disconnect the spark plug for storage

A disconnected spark plug means accidental pull-cord engagement cannot start the engine. Safer for children, safer for visitors who lean on the mower.

Reconnect in spring before the first start. First start should be cold-start cycle as if from new: choke if applicable, primer if applicable.

5. Store the mower properly

Indoor dry storage is best. Garage, shed, or barn. Avoid storing directly on concrete (cover the floor with cardboard or a pallet — concrete sweats moisture).

If outdoor storage is unavoidable, use a breathable mower cover (£15-£30) — not a plastic tarpaulin, which traps condensation underneath.

Battery removal on cordless — store the battery indoors at 40-60% charge in a dry place, not in the mower.