Why does my lawnmower cut out in hot weather?

This is a classic hot-weather fault and the pattern itself is the diagnosis: a machine that works for fifteen or twenty minutes, dies, refuses to restart, and then starts perfectly once it has stood for half an hour is telling you that something is failing at temperature rather than being broken. Cordless mowers cut out for related but different reasons, which is worth separating early.

Safety

Step by step

1. Note the pattern precisely.Cuts out after a consistent period of running and restarts only after cooling is a heat fault. Cuts out immediately under load in long grass is a different problem — usually a blunt blade, a blocked deck, or cutting too low. Never restarts at all is a fuel or ignition failure unrelated to temperature.
2. Clear the cooling path first.Air-cooled engines rely on fins around the cylinder and a fan drawing air over them, and dry grass clippings pack that space solid over a season. With the plug lead off, remove the engine cover and clean the fins and the fan housing out thoroughly. This is by far the most common cause and it costs nothing.
3. Clean or replace the air filter.A filter clogged with dry dust and clippings restricts air, runs the engine rich and hot, and summer is exactly when it clogs. Paper filters are replaced; foam filters are washed, dried fully and lightly oiled. Never run without one.
4. Check the deck underneath.Compacted grass under the deck loads the engine, blocks discharge and makes it run hot. Scrape it out properly with the plug lead disconnected, and check the blade is sharp — a blunt blade takes far more power to do the same work.
5. Consider vapour lock in the fuel.In high temperatures, petrol can vaporise in the fuel line or the carburettor, so the pump delivers vapour rather than liquid and the engine starves. Modern ethanol-blended fuel is more prone to it. Symptoms are exactly this pattern. Fresh fuel, a clear vent in the fuel cap, and keeping the fuel line away from hot engine parts all help.
6. Check the fuel cap vent.A tank has to admit air as fuel is drawn out. A blocked vent creates a vacuum that starves the engine progressively — it runs, dies, and then starts again once you loosen the cap and let air in. Loosening the cap when it next cuts out is the diagnostic test.
7. Suspect the ignition coil if the basics are clear.A failing coil often works cold and breaks down once warm, giving precisely this symptom. It is a common failure and a straightforward part to replace on most mowers, but confirm the cheap causes first — a coil is frequently blamed when the real problem was cooling fins packed with grass.
8. Use fresh fuel.Ethanol-blended petrol absorbs water and degrades within months, causing hard starting, gumming and poor running that is worse when hot. If the fuel has been in the can since last autumn, that alone may be the answer. Use fresh fuel and stabilise anything stored.
9. For a cordless mower, look at the battery instead.A battery mower cutting out in heat is usually thermal protection in the pack or the motor controller. Let it cool, keep the pack out of the sun between uses, rotate two batteries, and raise the cutting height so the motor works less hard. Never charge a hot pack.
10. Reduce the load in hot weather.Raise the cutting height, mow when the grass is dry but not in the hottest part of the day, take narrower passes in heavy growth, and let the machine rest. In a drought you should not be mowing short at all — long grass survives far better.

Tips

• Cleaning the cooling fins is the most-skipped mower maintenance job and the most common cause of this exact fault. Do it at the start of every season.
• A sharp blade reduces engine load noticeably. Sharpening once or twice a season is worth more than most other maintenance.
• In a drought, the correct answer is often not to mow at all. Long grass shades its own roots and survives far better.
• Do not tip a petrol mower on its side with the carburettor or air filter downward — oil and fuel flood into places they should not be. Check the manual for which way it tips.
• If the fault persists after cooling, filter, fuel and blade have all been dealt with, an ignition coil is a modestly priced part and a common failure. That is the point at which replacing it is a reasonable bet rather than a guess.

Common mistakes

• Replacing the spark plug and assuming that fixes a heat fault — A plug rarely causes a fault that appears only when hot. It is cheap to change and it usually is not the problem — the coil, the cooling path and the fuel are far more likely.
• Working on a mower without disconnecting the plug lead or battery — The blade can turn and a petrol engine can kick over. Disconnecting takes seconds and prevents serious hand injuries.
• Refuelling a hot engine — Petrol spilt onto a hot engine or exhaust can ignite. Let it cool first, and refuel on a hard surface rather than on grass.
• Mowing short in a drought to save doing it again — It removes the shade protecting the roots and is the fastest way to turn a dormant lawn into a dead one.

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