Few garden frustrations are as familiar as a petrol mower that fires up willingly, runs for three or four seconds, then dies as if you had switched it off. Pull again, same result: a brief eager burst followed by silence. The pattern itself is the diagnosis. A mower that starts and then stalls almost always has a fuel delivery problem rather than an ignition one, and in the large majority of cases the culprit is a blocked carburettor caused by old fuel. You can usually fix it at home in under an hour with a few pounds of carburettor cleaner and no special skills, and this guide walks through it from the quick checks to the full clean.
Why a Mower Starts Then Stalls
Understanding the timing tells you exactly where to look. When you prime the bulb or pull the choke, you push a small charge of fuel into the carburettor float bowl, the little reservoir under the carburettor. The engine starts on that primed charge and runs happily for a few seconds because the fuel is right there waiting. The trouble comes when that initial charge burns off and the engine needs to draw a steady supply through the main jet, a tiny calibrated hole that meters fuel into the airflow. If that jet is partly or fully blocked, no fresh fuel gets through, the engine starves, and it stalls. So the start-then-stall pattern is the signature of a clogged jet almost every time.
What blocks the jet is usually the fuel itself. Most petrol sold today contains ethanol, and ethanol is hygroscopic, meaning it draws moisture out of the air. Left in the tank over weeks or months, the fuel absorbs water and can separate into layers, a process called phase separation, leaving a watery non-combustible layer at the bottom and a gummy residue as the lighter components evaporate. Those gums and varnishes dry into the carburettor passages and shrink the main jet from a clear pinhole to a blocked or weeping one. This is why the problem so often appears on the first mow after winter, or after a mower has sat unused for a month in summer. The fuel went stale and the jet gummed up.
The Five Minute Checks Before You Touch the Carburettor
Before you take anything apart, rule out the simple causes, because two of them mimic a blocked jet and take seconds to check. First, the fuel itself. If the petrol in the tank is more than a month old, drain it into a suitable container and refill with fresh fuel. Stale petrol alone can cause the stall, and starting with fresh fuel sometimes solves the problem outright. Second, the fuel cap. Many caps have a tiny vent hole that lets air into the tank as fuel is used. If that vent clogs, a vacuum builds in the tank and chokes off the fuel supply after a few seconds, producing the exact start-then-stall behaviour. Test it by loosening the cap and trying to run the mower with the cap slightly open. If it keeps running, the cap vent is blocked and needs cleaning or replacing.
Next, glance at the air filter. A filter caked in dry grass and dust starves the engine of air and can cause stalling, and it is a two minute job to pop the cover, lift the filter out and either tap a paper one clean or wash a foam one in warm soapy water and let it dry fully before refitting. Finally, if you have an inline fuel filter, a clear plastic barrel in the fuel line, check that fuel is actually flowing through it and that it is not clouded with debris. If fresh fuel, a clear cap vent, a clean air filter and a flowing fuel filter have not fixed the stall, the carburettor is the problem and it is time to clean it.
How to Clean the Carburettor Step by Step
You will need a can of carburettor cleaner, which costs around £5 to £8 (about $6 to $10) for a 400ml aerosol from brands such as WD-40 or Gumout at Halfords, Screwfix, Amazon, Lowe’s or any motor factor, plus a basic socket or spanner set, a small screwdriver and a thin strand of wire such as a single copper core from electrical cable. Start safe: disconnect the spark plug cap so the engine cannot fire, and on most mowers tip it with the air filter side up so you do not flood oil into the cylinder. Turn off the fuel tap if fitted, or clamp the fuel line.
Locate the carburettor, usually behind the air filter housing, and remove the float bowl, the small cup held to the underside by a single central bolt. Have a rag ready, because residual fuel will spill. Inside that central bolt on many engines is the main jet itself, drilled through with the metering hole. Hold it up to the light. If you cannot see a clean point of light through the hole, it is blocked. Spray carburettor cleaner through the jet, then gently pass your thin wire through the hole to dislodge the gum, and spray again until you can see daylight clearly through it. Take the same care with the smaller pilot jet and any passages in the carburettor body, spraying cleaner through every drilling and into the area where the float sits. Check that the float moves freely and that the small rubber tipped needle it operates is not stuck, since a stuck needle stops fuel refilling the bowl.
Let the cleaner evaporate for a few minutes, then reassemble in reverse order, taking care not to overtighten the float bowl bolt, which can crush the jet or strip the thread. Reconnect the fuel, refit the spark plug cap, add fresh fuel if you have not already, and start the mower. A carburettor that was only lightly gummed will now run steadily. If it still stalls after a thorough clean, the jet may be too corroded to clear, in which case a complete replacement carburettor for a common engine costs only about £15 to £35 (roughly $15 to $40) and bolts on in fifteen minutes, often the better value option compared with a professional service.
Keeping It From Happening Again
Almost every start-then-stall fault traces back to fuel that sat too long, so prevention is mostly about fuel discipline. Buy petrol in quantities you will use within a few weeks rather than keeping a half full can for months. If you can find it, ethanol-free petrol, sometimes labelled E0 or sold as alkylate fuel for garden machinery, stores far longer and does not absorb water the way standard blends do, though it costs more. For everyday fuel, a fuel stabiliser added to the can slows the gumming process and is worth using through the season and especially before any spell of storage.
The single most useful habit is to run the carburettor dry before the mower sits idle for more than a couple of weeks. Turn off the fuel tap, or disconnect the line, and let the engine run until it stalls on its own. That empties the float bowl and jets so there is no fuel left inside to evaporate and form gum. A mower stored dry will usually start cleanly next time and run without the stall, while one left full of ageing fuel is the one you will be stripping the carburettor on next spring. Combine dry storage with fresh, stabilised fuel and a clean air filter, and the start-then-stall problem becomes something you fix once and then rarely meet again.
One mistake to avoid while diagnosing is reaching for the choke or primer to keep a stalling engine alive. Holding the choke on floods extra fuel into a carburettor that already cannot meter properly, which fouls the spark plug and masks the real fault without fixing it. If you find the mower will only run with the choke held partly closed, treat that as confirmation of a lean fuel supply through a blocked jet rather than a workaround, and clean the carburettor rather than mowing the whole lawn on choke. Working through the checks in order, fuel, cap vent, air filter, then carburettor, saves you from stripping parts that were never the problem.
