Why does my lawnmower start and then cut out?
Whether it dies after seconds, after minutes, or only under load tells you which system is failing, and each points at a different repair.
- Difficulty
- intermediate
- Time
- 45 min
- Read
- 5 min
- Safety
- warning
Short answer
Time it. Dying after a few seconds usually means it was running on the primer charge and fuel is not getting through. Dying after several minutes points to heat — a blocked tank vent, or an ignition coil breaking down when hot. Dying only in long grass is a cooling, air filter or blunt blade problem.
An engine that starts is proving it has compression and a spark at least some of the time, which removes a large part of the problem. What is left is the fuel supply, the ignition when hot, and whether the engine can breathe and cool itself. The timing of the stall is the most useful piece of evidence you have.
Safety
What you'll need
- Fresh petrol
- A spark plug spanner and a spare plug
- Carburettor cleaner
- Rags
- A new air filter (optional)
Step by step
- Note exactly how long it runs, and what you were doing.Seconds, minutes, or only under load in thick grass. Write it down. This one observation narrows the problem more than any test.
- If it dies within a few seconds, suspect fuel starvation.It ran on the fuel the primer put into the carburettor and then found nothing behind it. Check the fuel tap, the tank level, and whether the fuel is stale.
- Test the tank vent.Run it with the fuel cap loosened slightly. If it now keeps running, the vent hole in the cap is blocked and a vacuum is forming in the tank that stops fuel flowing. Clean the vent with a pin.
- Check for water in the fuel.Drain a little into a clear jar and let it settle. Ethanol petrol that has absorbed water separates into layers, with a cloudy or distinctly separated bottom. Water in the tank runs an engine for a few seconds at a time.
- Clean the carburettor jets.Old fuel leaves a varnish in passages the width of a pin. Carburettor cleaner sprayed into the intake and through the jets sometimes clears it. If not, the bowl needs removing and the main jet cleaning with a fine wire, which is where many people hand it to a service.
- If it dies after several minutes, suspect the ignition coil.A coil that is breaking down expands as it warms and loses its spark. The classic pattern is an engine that runs for five to fifteen minutes, dies, refuses to restart, and then starts perfectly once it has cooled for half an hour. That is a coil, and it is a replaceable part.
- Check the air filter properly.Hold it up to the light. If you cannot see light through a paper element, or a foam one is black and gummy, the engine is being throttled. Wash foam in warm soapy water, dry it fully, oil it lightly; replace paper.
- Clear the cooling fins and the recoil housing.Grass packed around the cylinder fins and under the starter cover stops the engine losing heat, and a hot engine stalls. It is a very common cause on a mower that has been used in long grass and never cleaned.
- If it only dies under load, look at the blade and the deck.A blunt blade, a deck packed with wet grass, or grass too long for the cut height all load the engine heavily. Raise the cut height, clear the deck with the plug lead off, and sharpen the blade.
- Check the oil level.Many four-stroke mowers have a low-oil cut-out that stops the engine. Low oil also causes overheating and permanent damage, so check it on level ground before anything else.
- Check the plug once more once you have a theory.Pull it after a stall. Wet with fuel suggests flooding or an ignition failure; bone dry suggests fuel starvation. It is a free confirmation of what you already suspect.
Tips
- Change one thing at a time and test. Replacing the plug, the filter and the fuel together tells you nothing about which it was.
- Never cut wet grass if you can avoid it — it packs the deck, loads the engine and makes every fault worse.
- A rough running engine that improves on part choke is telling you it is running lean, which usually means a partial blockage in the fuel system.
Common mistakes
- Assuming a stall after ten minutes is a fuel problem — A time-and-heat pattern that recovers after cooling is the signature of a failing ignition coil, not fuel. People replace plugs and clean carburettors repeatedly while the coil fails a little sooner each time.
- Ignoring grass packed around the cylinder fins — Air cooling is the only cooling an engine of this type has. Insulated by a mat of dried grass, it overheats and shuts down, and running it that way for long enough causes real damage.
If it doesn't work
Runs a few seconds each time it is primed
Cause: Fuel not reaching the carburettor — Fix: Check tap, vent and filter, drain stale fuel, then clean the carburettor jets.
Runs 10 minutes, stops, restarts only when cold
Cause: Ignition coil breaking down when hot — Fix: Replace the coil. It is a straightforward part on most small engines.
Stalls only in thick grass
Cause: Engine overloaded — blunt blade, packed deck or low cut height — Fix: Raise the cut height, clear the deck with the lead off, sharpen or replace the blade.
Engine dies and there is no oil on the dipstick
Cause: Low-oil cut-out, or oil loss — Fix: Top up to the mark with the correct grade, and find where the oil went before running it again.
Fuel looks cloudy or has two layers in a jar
Cause: Water absorbed by ethanol petrol — Fix: Drain the whole tank, dispose of the fuel properly, and refill with fresh petrol.
Questions people ask
Can I clean a carburettor without taking it apart?
Sometimes. Spraying carburettor cleaner into the intake while the engine runs, and through the jets with the bowl off, clears mild gumming. Heavy varnish from fuel left for a year needs the carburettor stripping and the jets cleaning individually.
How do I know if it is the coil without buying one?
The pattern is diagnostic on its own: consistent running time, a stall, no spark while hot, and a normal spark again once cool. Testing for a spark immediately after it stalls confirms it.