Stale fuel, nine times out of ten. Work through it in this order and most machines start before the end of the list.
1. Old fuel
The overwhelming answer. Petrol left from autumn degrades, and the residue blocks the very fine passages in a small carburettor.
Drain it. Properly — the tank and the float bowl, not just the tank. Refill with fresh, and try again.
2. The spark plug
Out, and look at it. Wet with fuel means it has been flooded — dry it, leave the machine a few minutes with the plug out, try again. Black and sooty means it has been running rich, which points back at the filter. Cracked porcelain or a badly eroded electrode means replace it, and it is a cheap part.
3. The air filter
Choked, and the engine cannot draw. See the filter page here for what each element type will tolerate.
4. The fuel tap and the primer
Tap open, if the machine has one. Primer bulb pressed the number of times the manual says — not fewer, which does nothing, and not many more, which floods it.
5. The safety bail
The operator-presence control has to be held against the handlebar for the engine to run at all, and a stretched or disconnected cable is a machine that turns over and dies. This is a real and frequently overlooked cause.
6. Oil level
Many engines have a low-oil cut-out that prevents starting. Check before concluding anything else.
7. The blade and the deck
A deck packed solid with last autumn’s grass can load the engine enough that it will not turn over freely. Clear it before diagnosing further.
When to stop
If it has fresh fuel, a good plug, a clean filter and the bail works, and it still will not fire, the carburettor needs cleaning. That is a bench job with the machine on its side, and on a machine you value it is worth paying for.
How this site handles it
The air filter shelf is the one this list sends people to most often, and its pages name the engine families the sellers name. Nothing on this site describes a machine as safe to work on; what the pages report is what each part is and what the listing says it fits.
The questions that keep coming in
How long does petrol actually last?
Long enough to be fine over a few weeks and not long enough to survive a winter untreated. What blocks a small carburettor is the residue left as it degrades, and the jets involved are very fine.
Is it worth paying for a service?
On a machine worth having, yes — a carburettor clean is straightforward work for someone who does it daily and fiddly for someone who does it once. On a cheap machine the service can approach the replacement cost.
Last reviewed 26 September 2026