Why will my petrol lawnmower not start?

An engine needs fuel, air, compression and a spark at the right moment. A mower that will not start is missing one of those, and there are only a handful of realistic reasons for each on a machine that worked last autumn.

Safety

What you'll need

• Fresh petrol
• A spark plug spanner
• A new spark plug, or a wire brush
• Clean rags
• A funnel (optional)

Step by step

1. Disconnect the spark plug lead before touching anything.Pull the rubber cap straight off the plug and tuck it away from the plug. Everything else in this guide happens with it disconnected, and it goes back on only when you are ready to start the engine.
2. Check the safety interlocks first, because they cost nothing.The dead-man handle must be held fully against the handlebar. Many mowers also have a key, a bail arm, or a switch that must be engaged. A mower with the handle not properly held will turn over and never fire, and people spend an afternoon on the carburettor first.
3. Check there is fuel, and check its age.Petrol degrades. Modern E10 petrol, with up to ten per cent ethanol, is worse than the old formulation: it absorbs water from the air and separates, and it starts losing usable life in as little as a month in a vented tank. Fuel from last autumn is the single most likely cause of a spring no-start.
4. Drain and replace stale fuel.Siphon or drain the tank into a proper fuel container, dispose of it at a household waste site rather than down a drain, and refill with fresh petrol. Do this outdoors, on a cold engine, away from any ignition source.
5. Check the fuel is actually reaching the engine.If there is a fuel tap, make sure it is on. Look for a blocked tank vent — undo the cap and try starting; if it starts with the cap loose, the vent is blocked. Some mowers have an inline filter that clogs.
6. Prime or choke as the machine expects.A primer bulb usually wants three firm presses; a choke lever wants to be fully closed for a cold start and opened as soon as it fires. Over-priming floods the engine, which is its own no-start.
7. Take the spark plug out and read it.Black and sooty means it has been running rich or is flooded. Wet with petrol means it is flooded. Pale and dry with a light tan colour is healthy. Oily means oil is getting past, which is a bigger problem.
8. Test for a spark.Refit the plug into its lead but rest its metal body firmly against a bare metal part of the engine, well away from the plug hole, and pull the starter. A strong blue spark means ignition is fine. Keep hands clear and do not do this if there is spilt petrol about.
9. Clean or replace the plug.A wire brush and a check of the gap will revive a serviceable plug; a plug that is cracked, heavily fouled or several years old is worth replacing outright. It is a cheap part and a common cause.
10. Check the air filter.A foam filter that is black and oily, or a paper one clogged with dust and grass, chokes the engine. Foam can be washed in warm soapy water, dried completely and lightly re-oiled; paper is replaced.
11. Clear a flooded engine.Remove the plug, hold the dead-man handle and pull the starter several times with the plug out to clear the cylinder — keeping clear of the plug hole — then dry the plug, refit it and try again with no choke or primer.
12. Deal with a gummed carburettor last.If it has fuel, air and a spark and still will not run, the small jets in the carburettor are almost certainly varnished up by old fuel. A carburettor cleaner sprayed through sometimes works; otherwise it needs stripping, which for most people is a job for a mower service.
13. Check the blade is not jammed before assuming the engine is seized.With the plug lead off, tip the mower with the air filter and carburettor side upwards and clear compacted grass from around the blade. A jammed blade prevents the engine turning over at all.

Tips

• Tip a mower with the carburettor and air filter uppermost. The other way, oil and fuel run into the filter and it will smoke and refuse to run afterwards.
• Buy petrol in small quantities and use it within a month or two, or add a fuel stabiliser when you buy it.
• Keep a spare spark plug in the shed. It costs little and it is the part most likely to end the problem.

Common mistakes

• Working near the blade with the plug lead connected — A single rotation of the blade can turn the engine enough to fire. It only takes one and it takes fingers. Pull the lead off first, every time, without exception.
• Persisting with last year's fuel — Ethanol-blended petrol absorbs water and forms gum in the carburettor's small jets. Once those jets are varnished, the machine will not run properly on fresh fuel either without cleaning, so a whole afternoon can be spent on a problem the first litre of fresh petrol would have prevented.

If it doesn't work

Questions people ask