What kind of gas does a lawn mower need?

What kind of gas does a lawn mower need?

A lawn mower needs fresh, regular unleaded petrol with an octane rating of 87 or higher, which is the standard grade at any fuel station. Premium high-octane petrol brings no benefit to a normal mower engine, so paying more for it is wasted money.

The two qualities that decide whether that fuel keeps your mower running are its ethanol content and its freshness, and both matter more than the octane number on the pump. Get those two right and almost any regular unleaded will serve a four-stroke mower well for years.

How much ethanol is safe in mower fuel?

Keep mower fuel to E10, meaning no more than 10 percent ethanol, which is what mower engines are designed to handle. Avoid E15 and E85, because their higher alcohol content corrodes fuel-system components and can void your engine warranty.

Ethanol is hygroscopic, meaning it attracts water, and in a small engine that moisture gathers in the carburettor and causes rough running, hard starting, and corrosion over time. Honda and Briggs & Stratton both caution against blends above E10 for exactly this reason. Where you can buy ethanol-free petrol, often sold in cans as small-engine or four-stroke fuel, it is the kindest fuel for a mower because it holds no water and stores for far longer than any ethanol blend, which makes it worth the extra cost for a mower used only occasionally.

Why does freshness matter more than the grade?

Freshness matters more than the grade because petrol begins to break down within roughly a month, long before octane is ever an issue. As it ages, the fuel oxidises and the ethanol draws in moisture, forming the gums and varnish that block a carburettor's tiny passages.

The practical rule is to buy only what you will use in about 30 days, so the fuel never has time to stale in the can or the tank. For any petrol that must sit longer, add a fuel stabiliser at the dose on the bottle, which extends its usable life for several months, or drain the tank before a long layup. A worked example: a mower filled in October and left untreated over winter often will not start in spring, whereas the same mower filled with stabilised fuel, run for a few minutes to circulate it, typically starts first time.

Four-stroke or two-stroke: which fuel does yours take?

Most modern walk-behind mowers are four-stroke, so they take straight petrol and hold their oil separately in the crankcase, and you never mix oil into the fuel. Only older or two-stroke mowers need a petrol-and-oil mix, so check your manual to be sure which you own.

The tell is simple: a four-stroke mower has a separate oil fill and dipstick, while a two-stroke engine has no separate oil and its manual quotes a fuel-to-oil ratio. Getting this wrong causes trouble in both directions, so it is worth a moment to confirm before the first fill. Owners who also run two-stroke garden tools should label their cans clearly to avoid pouring mixed fuel into the mower or straight petrol into a trimmer.

Does an oil-and-fuel mix ever apply?

An oil-and-fuel mix applies only to two-stroke mowers, which are now rare, while almost every modern walk-behind is four-stroke and takes straight petrol with its oil held separately. Mixing oil into a four-stroke mower's fuel makes it smoke heavily and run poorly.

If you are unsure, the manual settles it: a four-stroke engine has a separate oil fill and dipstick, whereas a two-stroke engine quotes a fuel-to-oil ratio such as 40:1. Owners who also run a two-stroke trimmer or blower should label their fuel cans clearly, so mixed fuel never ends up in the mower by mistake.

Storing fuel and topping up safely

Store petrol in an approved, sealed can in a cool, dry, well-ventilated place away from any appliance with a pilot flame, and keep only a month or two's worth on hand. Old fuel loses quality and becomes a disposal problem rather than a saving.

Refuel outdoors on a cool engine, never a hot one, and wipe up spills before starting. For fuel storage and the full seasonal service see our lawn mower maintenance guide, and for choosing a mower our gas lawn mower guide; the right fuel and oil supplies make the routine easy to keep up. Petrol is flammable, so handle it with care and always follow the fuel type your mower's manual specifies.