Portable Generator — Starts, Then Dies

Before you begin — carbon monoxide safety

Never run a generator indoors, in a garage, or in any enclosed space, even briefly for testing. Always test and run it outdoors, well away from windows and doors.

Overloaded — exceeding rated wattage

Most generators have an automatic overload shutdown to protect the engine and alternator. Note that appliances with motors (fridges, pumps, power tools) draw a much higher 'starting watts' spike than their running watts — this spike is a common hidden cause of overload trips.

1. Add up rated running watts of everything you want to run at once, and compare to the generator's rated continuous output.
2. Account for starting watts of any motor-driven appliance — this can be 2-3x its running watts for a brief moment.
3. Start large loads one at a time rather than all together, and unplug some items if you're over capacity.
4. Check for a reset button (often a red overload breaker) that may need manually resetting after a trip.

Low oil shutoff

Most generators have a low-oil sensor that shuts the engine down shortly after starting to protect it — very commonly mistaken for a mechanical fault, and this can trigger even if it started fine, since the shutoff activates once the engine is running and the sensor gets a reading.

1. Add oil to the full mark using the weight in your manual (commonly SAE 30 or 10W-30).
2. Make sure the generator is on level ground when checking — an unlevel surface gives a false low reading.
3. If oil was very low, check for a leak before running under load again.

Parts that may help: SAE 30 / 10W-30 small engine oil, small funnel

Stale fuel gumming the carburetor

Standby generators often sit for months, and stale fuel gumming the carburetor's idle circuit is a very common cause of starting briefly then dying, since it can run on a rich initial charge before the restricted idle circuit takes over.

1. Drain the tank and carburetor bowl completely into an approved container.
2. Refill with fresh gasoline.
3. If it still dies after fresh fuel, the carburetor jets likely need cleaning — expect this after long storage.
4. Going forward: add fuel stabilizer before storage, or run the tank dry before putting it away.

Parts that may help: e.g. Sta-Bil, Sea Foam, carburetor/choke cleaner spray

Blocked fuel cap vent

A clogged vent creates a vacuum in the tank as fuel is drawn out, starving the engine after a short run.

1. Clean the vent hole with a pin or compressed air.
2. Replace the cap if cleaning doesn't help.
3. Confirm the fix running with the cap properly tightened.

Parts that may help: engine-model-specific vented fuel cap

Clogged air filter

A blocked filter starves the engine, especially common on generators stored in dusty sheds or garages.

1. Clean or replace per your model's filter type.
2. Check it at the start of each season, even if unused.

Parts that may help: engine-model-specific air filter

Ignition coil failing under heat (thermal breakdown)

Some failing ignition coils test fine cold but break down electrically once they reach normal operating temperature. This shows up as an engine that starts fine cold but dies sooner on each subsequent warm restart.

1. If you have a spark tester, checking spark strength once the engine is warm versus a strong cold-start spark can help confirm this.
2. Given the intermittent nature of this fault, replacing the coil outright is often more practical than extended testing.

Parts that may help: engine-model-specific ignition coil

Damaged carburetor float or needle valve

A stuck or waterlogged float lets the bowl run dry after a short time even though it seemed full initially — very common on generators after long storage periods.

1. Replace the float and needle valve as a set — sold together in most carb rebuild kits.
2. Clean all jets and passages with carb cleaner spray while the bowl is off.
3. Check the float sits level per your manual's spec before closing the bowl back up.

Parts that may help: engine-model-specific carburetor rebuild kit

Needs professional diagnosis

You've ruled out load, oil, fuel freshness, fuel cap, air filter, coil heat-soak behavior, and carburetor float. Remaining causes are specific enough to your engine that a shop's equipment will get there faster than continued guessing.

1. This is a good candidate for a local small engine shop — bring your notes on what's already been ruled out.

If this doesn't resolve it, this is a good candidate for a local small engine shop rather than continued DIY diagnosis.