Lawn Mower Starts Then Dies After a Few Seconds

Quick Answer

A lawn mower that starts then dies after a few seconds is almost always a fuel delivery problem. The engine gets enough fuel to start (from the primer bulb or choke enrichment) but cannot sustain combustion because fuel is not flowing to the carburetor consistently. The three most common causes are a clogged carburetor (from old fuel leaving varnish deposits), a dirty fuel filter, or a blocked fuel cap vent. Start by draining old fuel and replacing it with fresh gas, then clean or replace the carburetor if the problem persists.

Understanding What "Starts Then Dies" Tells You

This symptom is actually very informative. A mower that refuses to start at all could have ignition problems, compression problems, or fuel problems. But a mower that starts, runs for 2-5 seconds, and then sputters out has narrowed the diagnosis for you -- the ignition system is fine (the spark plug is firing), and the engine has compression (it is producing power). The problem is fuel delivery.

When you press the primer bulb or engage the choke, you are manually pushing a small amount of fuel directly into the carburetor's intake. That shot of fuel is enough to start the engine. Once running, the engine needs a continuous flow of fuel through the carburetor's jets and passages. If something is restricting that flow, the engine burns through the initial primer fuel and starves.

Think of it like lighting a campfire with lighter fluid -- the fluid catches and burns, but if the wood is too damp to sustain the flame on its own, the fire dies as soon as the lighter fluid is consumed.

Most Likely Cause: Stale Fuel and a Gummed-Up Carburetor

This is the culprit in the majority of cases, especially after winter storage. Gasoline starts to degrade after about 30 days. Over a few months, volatile components evaporate, leaving behind a varnish-like residue. This residue coats the inside of the carburetor, clogging the tiny jets and passages that meter fuel into the engine.

The main jet -- a small brass orifice about the diameter of a needle -- is particularly vulnerable. Even a thin film of varnish on this jet can restrict fuel flow enough to starve the engine at running speed while still allowing enough fuel through for a brief start.

If your mower sat over winter with fuel in the tank and you did not use a fuel stabilizer, this is overwhelmingly the most likely problem.

The fix: Drain the old fuel from the tank. Remove the carburetor (on most push mowers, it is held on by two bolts and connected by a fuel line and throttle linkage). Disassemble it and soak the body, bowl, and jets in carburetor cleaner for an hour. Use a thin wire or a carburetor cleaning kit needle to clear each jet orifice. Reassemble, reinstall, fill with fresh fuel, and try again.

If you are not comfortable disassembling a carburetor, replacement carburetors for most common push mower engines (Briggs & Stratton, Honda, Kohler) are available online for fifteen to twenty-five dollars and bolt directly in place of the old one. Sometimes replacement is easier and cheaper than cleaning.

Second Most Likely: Blocked Fuel Cap Vent

This one catches people because it seems too simple. The fuel cap on most mowers has a small vent hole that allows air into the tank as fuel is consumed. Without this vent, the engine creates a vacuum in the tank as it draws fuel, and eventually the vacuum becomes strong enough to stop fuel flow entirely.

The signature symptom of a blocked vent is that the mower starts, runs for 5-30 seconds, then dies. If you loosen the fuel cap and the mower immediately restarts and runs normally, you have found the problem.

The vent can be blocked by dirt, grass clippings, or spider webs (spiders love small holes on outdoor equipment). Clean the vent with a thin pin or replace the cap.

Other Common Causes

Dirty or clogged fuel filter. Inline fuel filters can get blocked by debris or old fuel residue. If your mower has one (check the fuel line between the tank and the carburetor for a small clear or opaque cylinder), replace it. They cost a dollar or two.

Blocked fuel line. Disconnect the fuel line from the carburetor and hold it over a container. Fuel should flow freely by gravity. If it drips slowly or not at all, the line is blocked. Replace it with a new piece of fuel line from the hardware store.

Old or incorrect fuel. Gasoline with more than 10% ethanol (E15 or E85) can cause running problems in small engines that are not designed for it. Use regular unleaded E10 or ethanol-free fuel if available. Also, two-stroke mowers require premixed fuel with oil -- running straight gasoline in a two-stroke will cause it to seize.

Faulty carburetor gaskets. The gaskets between the carburetor, the intake manifold, and the engine block can crack or shrink over time, creating an air leak. An air leak leans out the fuel mixture (too much air, not enough fuel) and can cause the engine to start but not sustain idle. If you are already removing the carburetor, replace these gaskets -- they are included in most carburetor rebuild kits.

Choke not disengaging. On mowers with an automatic choke, a stuck choke plate can cause the engine to flood after starting (too much fuel) or starve (too little fuel) depending on which position it is stuck in. Check that the choke plate moves freely and that the choke linkage is not bent or disconnected.

Seasonal Maintenance That Prevents This

Almost every "starts then dies" problem can be prevented with end-of-season maintenance. Before storing your mower for winter, do the following.

Run the engine until the fuel tank and carburetor are empty. This eliminates the stale fuel problem entirely. Alternatively, fill the tank with fresh fuel and add fuel stabilizer according to the label directions, then run the engine for a few minutes to circulate the stabilized fuel through the carburetor.

Replace the spark plug annually. A new plug costs three to five dollars and ensures reliable ignition every spring.

Change the oil. Old oil with combustion byproducts is mildly acidic and can cause internal corrosion during storage.

Clean or replace the air filter. A dirty air filter restricts airflow to the carburetor, and a wet or oily filter can cause running issues similar to fuel problems.

These are the kinds of small maintenance tasks that, like checking your dryer's drum belt before it breaks, save you a much bigger headache later.