A petrol mower that will not start after months in the shed almost always has one of five faults, and you can fix most of them in half an hour with cheap parts. Work through them in order of likelihood: stale fuel first, then the spark plug, then the air filter, then the carburettor, and finally the oil level and safety cut-outs. Tackling them in that sequence saves you from stripping a carburettor when the real problem was old petrol.
Start With the Fuel, the Most Common Cause
Petrol does not keep. It begins to degrade in as little as 30 days, and fuel containing ethanol, which most pump petrol now does, is worse. Ethanol attracts and absorbs moisture from the air. Over a few months the fuel can separate, with a watery ethanol layer sinking to the bottom of the tank, a process called phase separation. What is left turns to a sticky varnish or gum that clogs the fine passages in the carburettor. If your mower sat all winter on last year’s petrol, suspect the fuel before anything else.
Drain the old fuel from the tank and dispose of it safely, then refill with fresh petrol and try again. A large share of no-start mowers come back to life on fresh fuel alone. To prevent the problem next time, add a fuel stabiliser such as STA-BIL (around 10 to 12 pounds / 10 to 13 dollars a bottle, treating many tankfuls) to the petrol before storage, which keeps it usable for up to two years, and store the mower with a full tank to reduce the air space where moisture can condense. Where you have the choice, the lower-ethanol grade keeps better than the higher-ethanol one.
Spark Plug and Air Filter
If fresh fuel does not do it, pull the spark plug. Use a plug spanner to remove it and look at the tip. A plug coated in black soot, oily film or wet fuel cannot produce a strong enough spark to fire the engine. Clean the electrode with a wire brush, check the gap is around 0.75mm (0.030 inches) with a feeler gauge, and refit it. Spark plugs are cheap, around 4 to 6 pounds / 5 to 8 dollars for a name brand such as NGK or Champion from Screwfix, Amazon, Home Depot or any motor factor, so if the old one looks worn, just replace it. A new plug each season, or every 25 running hours, is good practice and rules the plug out as a suspect.
Next check the air filter. The engine needs a steady flow of clean air to burn fuel, and a filter choked with dust and clippings starves the combustion of oxygen, so the mixture is too rich to fire. Filters are either a paper cartridge or a foam element. Tap a paper filter out and hold it to the light: if you cannot see light through it, replace it (around 6 to 8 pounds / 8 to 10 dollars). Wash a foam filter in warm soapy water, rinse it, let it dry completely, and refit it lightly oiled if the manual says so. Clean the filter roughly every 25 hours of use, and more often in dusty, dry conditions like a summer mow.
Carburettor, Oil and Safety Cut-Outs
If the engine turns over, maybe fires briefly, then dies, the carburettor is the likely culprit, gummed up by the old fuel described earlier. Spray carburettor cleaner (around 6 to 8 pounds / 8 to 10 dollars a can) into the air intake and try starting. For a heavier blockage, remove the float bowl underneath the carburettor, clean out any varnish and debris, and clear the small main jet with cleaner and, if needed, a fine wire. Ethanol gum in the jet is the usual reason an engine starts on a squirt of fuel but will not keep running on its own.
Two simple things catch people out. First, the oil level: many modern mowers have a low-oil sensor that blocks the engine from starting to protect it. Check the dipstick and top up with the grade in your manual, commonly SAE 30 or 10W-30, before assuming the worst. Second, the safety cut-outs: the operator presence bar on the handle must be held against the grip for the engine to run, the spark plug cap must be pushed on firmly, and any blade-brake or kill switch must be in the right position. A mower that gives no response at all often has nothing more wrong than a loose plug cap or a bail lever that is not being held.
If you have worked through fuel, plug, filter, carburettor, oil and safety switches and the engine still will not run, the fault may be deeper, such as low compression or a failed ignition coil, and at that point it is worth weighing the cost of a service against a replacement. Our guide on when to repair an old mower and when to replace it walks through that decision so you do not pour money into a machine past saving.
Choke, Primer and a Flooded Engine
Two starting controls trip people up at the first pull of the season. The choke restricts air to give a richer fuel mixture for a cold start, and it must be set to the closed or full-choke position when the engine is cold, then opened once it fires and warms up. Leaving the choke open on a cold engine often means it will not catch at all; leaving it closed after the engine is warm floods it. The primer bulb, the small rubber dome on the side of many smaller engines, squirts a shot of fuel into the carburettor. Three firm presses is usually right. Pumping it ten times pushes in far too much fuel and floods the engine.
A flooded engine has too much fuel and not enough air to ignite, and you can usually smell raw petrol around the deck. The fix is patience: open the choke fully, move away from the primer, and pull the cord several times to clear the excess, or remove the spark plug, wipe it dry, leave the cylinder open for ten minutes to air, then refit and try again. Forcing repeated pulls on a flooded engine only wets the plug further and wears out your shoulder.
It is also worth a quick look at the fuel actually reaching the engine. Check that any fuel tap or shut-off valve under the tank is turned on, because it is easy to leave it closed after winter storage. Inspect the fuel line for cracks or kinks, and if your mower has an inline fuel filter, hold it to the light to see whether it is clogged with debris or gum; a blocked filter starves the carburettor just as old fuel does. On electric-start petrol mowers, a flat starter battery will leave you with nothing happening when you turn the key, so charge or replace the battery before suspecting the engine.
Before you store the mower at the end of the season, a short routine prevents most of next year’s no-start headaches: either run the tank dry or add stabiliser and run the engine for a few minutes so treated fuel reaches the carburettor, change or clean the oil, take out and clean the spark plug, and wash the air filter. A clean, fuel-stabilised mower put away in autumn almost always starts first or second pull when you wheel it out in spring, which is the cheapest fix of all because it avoids the problem entirely.
Worked in order, the whole check rarely takes more than half an hour and costs very little. Fresh petrol revives the largest share of dead mowers on its own. A clean or new spark plug and a clear air filter account for most of the rest. A squirt of carburettor cleaner, the right oil level, and a properly held safety bar with a firm plug cap usually cover what is left. Keep a spare spark plug, a spare air filter and a can of carburettor cleaner in the shed, total cost around 15 to 20 pounds / 20 to 25 dollars, and you can work through every common fault in one session without a trip to the shops. Only when all of that draws a blank is it time to think about a deeper engine fault or a new machine.
