Why does my mower start then die?
Updated August 2026
A mower that fires up, runs for 30–60 seconds, then quits is nearly always starved for fuel by a gummed carburetor or a blocked fuel-cap vent: the bowl holds just enough gas to start, then nothing refills it and the engine stalls. Begin with the two-second test, loosen the fuel cap and restart. If it keeps running, the vent was the culprit. If not, stale fuel and a varnished carburetor are next, and the causes below run roughly from most to least common.
Safety first: gasoline is highly flammable and its fumes travel along the ground, so refuel a cool engine outdoors, never near a pilot light or a cigarette, and wipe up spills before you pull the cord. Before you reach anywhere near the blade or tip the deck to inspect the carburetor, pull the spark-plug wire so the engine cannot fire on you.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| Gummed carburetor from stale fuel | Most common | Drain the old gas, spray the jets with carburetor cleaner or fit a rebuild kit, then refill with fresh fuel. | $0–80 |
| Blocked fuel-cap vent | Common | Loosen the cap and restart; if it keeps running, clean the vent hole or replace the cap. | $0–15 |
| Dirty air filter or stuck choke | Common | Replace a clogged paper or foam element and free a choke linkage that is stuck closed. | $8–15 |
| Clogged fuel filter or kinked line | Occasional | Swap the inline fuel filter and straighten or replace a cracked, varnished fuel line. | $5–20 |
| Fouled or worn spark plug | Occasional | Pull the plug, clean or replace it, and set the gap to the engine spec. | $4–10 |
| Weak ignition coil or sheared flywheel key | Rare | Have the coil tested and replaced, or fit a new flywheel key if the timing slipped. | $40–120 |
Check these first
- Loosen the fuel cap and restart. If it now keeps running, the cap vent is plugged and pulling a vacuum on the tank.
- Check the age of the gas. Fuel older than about 30 days, especially E10 with ethanol, gums the jets; drain it and refill with fresh.
- Pull the air filter and hold it to the light. A clogged paper or oil-soaked foam element chokes the engine within a minute.
- Confirm the choke opens as the engine warms. A choke stuck closed floods the carburetor and stalls it after it fires.
- Check the fuel shutoff valve and inspect the inline fuel filter for varnish or debris.
Prevent it next time
Nearly every start-then-die is a fuel-quality problem in disguise, so the habit that prevents it is storage discipline: at the end of the season, either run the tank dry or add stabilizer, which is the heart of a proper winterizing routine. During the season, buy only as much gas as you will burn in 30 days, keep the spark plug fresh, and change the oil on schedule so a clean engine is not fighting a dirty one.
When to call a pro
Call a small-engine shop when fresh fuel, a clean filter, and a carb spray have not brought it back within an hour of tinkering, or when it still dies with a new plug and a clear fuel path (a sign of a weak ignition coil or a sheared flywheel key). A shop diagnostic runs $50–100, a carburetor rebuild or replacement $60–150, and a coil about $40–120. On a mower worth $250–500 new, that math is usually worth it once; twice in two years, replace it.
Most of the causes above are maintenance problems wearing a disguise. OnOtto keeps the boring prevention on schedule so the 2 a.m. version doesn't happen.
Stop chasing symptoms, free for 30 daysFrequently asked questions
How do I tell if it is the carburetor or the fuel cap?
Run the mower, and the moment it starts to die, loosen the fuel cap. If the engine recovers and keeps running, the cap vent is plugged and starving the tank of air; a $5–15 replacement cap fixes it. If loosening the cap changes nothing and the engine still quits after 30–60 seconds, the carburetor jets are gummed with old fuel. Drain the stale gas, spray the jets with carb cleaner, and refill with fresh. Two-minute test, and it tells you which repair to make.
Can old gas really make a mower start then die?
Yes, and it is the single most common cause. Pump gasoline with ethanol (E10) starts to break down in about 30 days and draws in moisture, leaving a varnish that clogs the carburetor's tiny jets. There is often just enough fuel in the bowl to fire the engine for a few seconds, then it starves and stalls. Drain anything older than a month, refill with fresh gas, and add stabilizer if the mower will sit. If the varnish has already hardened, the carb needs a cleaning or a $10–25 rebuild kit.
Should I clean or replace the carburetor?
Clean it first. A can of carburetor spray costs a few dollars, and blowing out the jets fixes most start-then-die engines. If cleaning does not hold, a rebuild kit with fresh gaskets and a needle runs $10–25 and suits an afternoon of work. A complete replacement carburetor for a common push-mower engine is only $30–80, so if the body is corroded or the throttle shaft is worn, swapping the whole unit is often faster than fighting it. Save the pro visit for engines you cannot source parts for.
Why does it die only after it warms up?
A hot stall points to two suspects. A blocked fuel-cap vent builds vacuum as fuel is drawn down, and the engine quits once the tank cannot feed; loosen the cap to confirm. The other is a failing ignition coil that works cold but breaks down as it heats, cutting spark after a few minutes of run time. Test the cap first because it is free, then have the coil checked (about $40–120 installed) if the vent is clear and the fuel is fresh.