- Petrol left in a mower over winter pulls in water and leaves gum in the carburettor, which can stop it starting in spring.
- A cheap hand siphon pump empties the tank safely, so you never need to tip the mower over.
- Fresh petrol with stabiliser already mixed in is the other route if your manual allows it, and stabiliser cannot bring old fuel back to life.
Why Old Petrol Can Stop Your Mower Starting in Spring
You put the mower away with a tank of petrol, and in spring it coughs, splutters or does nothing. Stale fuel is a leading cause of mower breakdowns. The fix takes under 30 minutes: siphon the tank into a proper fuel container, run the engine until it stalls so the carburettor empties too, then store the mower dry. Or refill with fresh petrol that already has stabiliser in it, if your manual says that is the way.
Here is what goes wrong when you do neither. David Bush, assistant manager of shows, events and sponsorship at Honda Power Sports & Products Division, explains that most mower petrol contains ethanol, an alcohol that attracts moisture. “Tiny amounts of water are generally ok, but larger quantities will likely cause corrosion (rust) and could clog the fuel system or carburetor,” he says. The result is a mower that will not start or runs roughly, and a repair bill.
The mechanism is simple. Ethanol blended petrol can hold only a limited amount of water in solution. Past that limit, the mix of ethanol and water separates from the petrol. It is denser than petrol, so it settles to the bottom. Bell Performance, a maker of fuel additives, adds that the carburettor bowl holds a small reservoir of fuel the entire time the mower sits, so that tiny pool soaks in whatever water the fuel has gathered. Water can contribute to corrosion of aluminium and other metal parts, and it does not take much contamination to interfere with the amount of fuel passing through a carburettor jet.
Timing is shorter than most people expect. The drain page notes that timings vary with the blend, the amount in the tank and the temperature, but petrol can go bad within a month of the mower sitting idle. Bell Performance puts the start of deterioration at as little as 30 days. A mower parked from late autumn until spring sits for far longer than that. If your mower already refuses to start, a mower that will not start has a short list of cheap checks worth running in order before you pay anyone.
Drain It or Stabilise It: How to Choose
Makers disagree, so your manual decides. The winterising page at Family Handyman notes that manufacturers such as Honda and John Deere recommend different ways to store their products: draining the petrol completely, or filling the tank with fresh, stabilised petrol. Read the storage section of your manual first. It takes two minutes and settles the question for your engine.
There is a third route. Run the mower dry on the last cut of the season. This is convenient when the tank is nearly empty, but it is neither economical nor kind to the environment when the tank is full.
If the manual allows stabiliser, the rule to remember is that it goes into fresh fuel only. The winterising page states it flatly: never add fuel stabiliser to old gas, as it will not bring it back to life. Xavier Sabastian, an author and auto expert with Way.com, says the best moment to add it is when you fill up. “Adding the stabilizer to fresh fuel ensures maximum effectiveness in preserving fuel quality during storage,” he says.
Stabiliser slows the clock. It does not stop it. Chris “Moose” Pyle makes the point bluntly. “Know that this is not a miracle additive, however,” says Chris “Moose” Pyle, an automotive expert with JustAnswer. “It only helps slow the aging process and it will only provide a couple more months. Fuel that is three or 30 or 300 days old will burn, but newer fuel will burn cleaner and easier, ignite better and leave behind less varnish and carbon.”
Claims about how long stabilised petrol lasts do not agree. The drain page says a stabiliser keeps petrol from going stale for at least 12 months. The stabiliser page says stabilised petrol can be stored for up to 12 months but should be used within 6 to 8 months. Pyle says a couple more months. The label on one brand, listed by Frost, gives a dose of 30 ml (1 fl oz) per 9.5 litres (2.5 gallons) of fresh fuel for 1 to 12 months of storage, and double that for 12 to 24 months. Treat the shortest figure as the safe one, and let your manual break any tie.
Which bottle? Pyle recommends the STA-BIL brand. “It is most popular, very effective and has been around for a long time,” he says. “No need to search for anything better or try other brands.” A 236 ml (8 fl oz) bottle is listed at £9.99 including VAT at Frost (out of stock when checked on 14 November 2026) and at $7.59 at Overton’s, where it treats up to 20 gallons (75.7 litres). Sabastian’s advice for any brand: “Look for products that offer protection against gum and varnish buildup, corrosion and phase separation in ethanol-blended fuels,” he says. The stabiliser page adds that most stabilisers work with all grades of petrol, including ethanol blends, and that you should check the label to be sure. The amount for a mower tank is small, so measure the dose from the label for the quantity of fuel you are treating.
How to Empty the Tank Step by Step
Start with safety. Petrol is the dangerous part. “Anytime you’re handling gasoline, a risk is present,” Bush says. It is highly flammable and the vapours are toxic.
“Always perform the task in a well-ventilated area,” Bush says. Open the garage door or work outside, away from pilot lights and anything that sparks. Turn the mower off, let the engine cool and disconnect the spark plug lead, so the engine cannot fire while your hands are near it.
“Wear proper gloves, eye protection, long-sleeved clothing and pants” Bush says. That covers you against skin-irritating splashes. Keep paper towels to hand and wipe up any spill at once. Spills can harm health and damage surfaces.
You need two things: an approved fuel container and a hand siphon pump. The drain page calls an inexpensive siphon pump the best tool for the job, so check the price at your local store. Lay everything out on a flat, clear patch of ground and read the manual for model-specific guidance before you begin.
“It offers good control when transferring fuel into a container, helping to prevent spillage,” Bush says of the siphon pump. Work through it in this order:
- Unscrew the petrol cap.
- Place the pump’s suction end in the tank and its outlet end in the fuel container.
- Prime the pump by pressing the handle several times, then wait until the fuel stops flowing into the container.
- Shake the hose to get the last drops out and wipe it dry with paper towels so it does not drip.
- Seal the container straight away.
Do not tilt the mower to empty it. Tipping can flood the carburettor, soak the air filter and spill fuel. The winterising page does describe tipping the mower over a drain pan, so you might meet that advice elsewhere, and some manuals describe it for their own machines. Check yours. The advice here is to siphon: a flooded carburettor and a soaked air filter are the faults you are trying to avoid.
Next comes the small amount left in the fuel lines and the carburettor bowl. Reconnect the spark plug, start the engine and let it run until it stalls. Shawn Carothers, senior marketing manager at Toro, gives the test: “Continue to attempt to restart the engine until it no longer starts.” Pull the cord or press the starter until nothing happens. That fuel is the pool that gums the jets, so getting rid of it counts for more than the last drops in the tank. If your carburettor has a drain valve or bolt on the bowl, the winterising page says to open it and drain the fuel into a container, and to run the engine until it stalls only if there is no valve or bolt.
To store the mower with stabilised fuel instead, empty the old petrol first. Pour stabiliser into an empty fuel container, then fill the container with fresh petrol at the pump so the two mix thoroughly. Fill the mower tank, run the mower for 10 minutes so the carburettor bowl fills with stabilised fuel, top off the tank and store the mower. Skipping the run leaves untreated petrol sitting in the bowl.
What to Do With the Old Petrol
Old petrol does not go in the household bin, down a drain or onto the ground. Carothers explains that you cannot throw it in your regular rubbish. “Follow your city’s guidelines for disposing of hazardous waste,” she says. The details are usually on your local council or city website, and many recycling centres take it. Keep it sealed in the approved container until you drop it off.
How to Stop It Coming Back, and When to Call Someone
Make the last cut of the season with a low tank, so there is less to siphon. Every spring, start with fresh petrol rather than what is left in a can from last year. Stabiliser is cheap insurance for the fuel you keep in storage, provided it goes into fresh petrol.
If the mower still struggles to start after fresh fuel, the cause could sit elsewhere, and a fuel filter in the line is a cheap part to rule out before anyone strips a carburettor.
Some jobs belong to a professional. Vince Jagodzinski, corporate grounds manager at Stanley Black & Decker, suggests hiring one when:
- you cannot properly follow the recommendations in the manufacturer’s instructions
- you do not have space to move around safely
- you cannot dispose of the spent fuel safely
- you have other mower maintenance or repair jobs that a professional could combine with this one
- your mower is under warranty and accidental damage could void it
Jagodzinski adds that draining is also needed when you carry a mower in a vehicle, to prevent damaging, flammable spills. Do the job once, do it calmly, and the mower you pull out of the shed in spring should start in a few pulls.
