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If your generator will not start, the cause is stale fuel in the great majority of cases — specifically, varnish blocking the carburetor's main jet. Before anything else, confirm three things in ten seconds: the fuel valve is open, the choke is closed for a cold start, and the engine has enough oil, because the low-oil sensor will refuse to start the engine no matter how good the fuel and spark are. Then work down the list below in order. It is arranged by how often each fault actually turns out to be the answer, not by how interesting it is.
The thirty-second checks
An embarrassing share of no-start calls end here, so do these before you touch a tool.
- Fuel valve open. Many machines have both a valve and a separate vent on the fuel cap. Both must be open.
- Fuel cap vent open. A closed vent creates a vacuum in the tank that starves the carburetor within a minute or two of running.
- Choke closed for a cold engine, open for a warm one. A warm engine flooded by an unnecessary choke behaves exactly like a no-fuel fault.
- Oil at the correct level, machine on level ground. The low-oil shutdown does not care whether the level is truly low or the generator is on a slope.
- Everything unplugged. A generator asked to start against a load will often refuse. Disconnect every cord and switch the breakers off before cranking.
- Engine switch on. On many machines the run switch, the fuel valve and the choke are three separate controls and one is easy to miss.
Fuel: the cause of most no-starts
Gasoline sold in the United States generally contains up to 10 percent ethanol, and it starts to degrade within roughly 30 days. Two things go wrong. Ethanol absorbs water from the air, which separates out and sits at the bottom of the tank, and the lighter fractions of the gasoline evaporate, leaving behind a sticky varnish. That varnish collects in the smallest passage in the fuel system — the carburetor main jet, which is a hole roughly the diameter of a pin — and blocks it completely.
The signature is distinctive: the engine cranks strongly, the plug is wet or dry depending on how long it has sat, there is fuel in the tank, and nothing happens. A quick diagnostic is a small squirt of starting fluid into the air intake. If the engine fires for a second and dies, you have confirmed spark and compression, and the fault is fuel delivery. Use starting fluid sparingly — it is a diagnostic, not a running strategy, and repeated use is hard on an engine.
The fix, in order of effort:
- Drain the tank and the carburetor bowl completely. Dispose of old fuel properly; most municipal household hazardous waste sites take it free.
- Refill with fresh gasoline. If your machine is dual fuel, try running it on propane — propane does not go stale, and a machine that runs perfectly on propane and not on gasoline has just told you exactly where the fault is.
- Remove the carburetor bowl (usually one bolt underneath) and check for a sludge or a green-brown residue. Clean the bowl and spray carburetor cleaner up through the main jet.
- Remove the main jet and clear it with a single strand of copper wire from a piece of stranded cable. Do not use a drill bit or a needle — enlarging that hole permanently changes the fuel mixture.
- If the passages are heavily varnished, a complete replacement carburetor for a common portable generator costs roughly $25 to $60 and is generally a four-bolt job.
Spark and the rest of the ignition side
If starting fluid produces nothing at all, the problem is spark or compression rather than fuel. Pull the spark plug and look at it. A plug that is soaked black and wet indicates flooding — leave the plug out, hold the throttle open and crank a few times to clear the cylinder, then fit a dry plug. A plug with a light tan deposit is healthy. A plug with a heavy carbon crust, an eroded electrode or a cracked ceramic gets replaced; they cost a few dollars and a generator that sits for years deserves a fresh one anyway.
To confirm spark, fit the plug back into its lead, hold the threaded body firmly against a bare metal part of the engine, and crank. A strong blue spark is right; a weak orange one or none at all points at the coil, the kill switch wiring, or a low-oil sensor that is cutting ignition. On many designs the low-oil sensor works by grounding the ignition, which is why an oil problem presents as a spark problem.
It runs but produces no power
A generator that starts and runs normally but delivers nothing at the outlets is a different fault family, and the order below finds it fastest.
| Check | What it looks like | Fix |
|---|---|---|
| Main breaker tripped | Zero volts at every outlet | Reset it. If it trips again immediately, you have a short or an overload |
| GFCI outlet tripped | Dead outlets, working twist-lock | Press reset on the outlet itself |
| Faulty cord or tester | One outlet dead, others fine | Test with a known-good cord and a plug-in outlet tester |
| Lost residual magnetism | Engine perfect, every outlet reads zero | Field flashing — a shop job unless you are experienced |
| Failed voltage regulator (AVR) | Very low or wildly unstable voltage | Replace the AVR, typically $30–$80 |
| Worn brushes | Output fades in and out | Replace brushes, usually inexpensive |
| Failed stator or rotor winding | No output, no response to any fix | Often uneconomic on a budget machine |
Residual magnetism is the one that surprises people. An alternator needs a small permanent magnetic field in its iron to begin producing current at all, and that field can decay during long storage or after the machine was shut down with a load still connected. The generator then spins perfectly and produces nothing. Restoring it — "flashing the field" — involves briefly energizing the winding while the engine runs, and it means working around live windings on a running machine. It is a routine job for a small-engine shop and a genuinely poor first electrical project. The habit that prevents it: always switch the breakers off and unplug everything before shutting the engine down.
It surges, hunts, or will not hold speed
Surging is a rhythmic rise and fall in engine speed, usually once or twice a second, and it means the governor is chasing a target it cannot hold. Nine times out of ten the underlying cause is fuel starvation from a partially blocked carburetor — enough fuel gets through to run, not enough to sustain the load, so the governor opens the throttle, gets a brief surplus, closes it, and repeats.
Work through these in order:
- Stale fuel and a partly blocked jet. Same fix as a no-start; the jet is partially rather than fully obstructed.
- Blocked fuel cap vent. Loosen the cap while it surges. If it settles, the vent is blocked and the fix is cleaning it.
- Clogged fuel filter or a kinked line. Inline filters are cheap and often forgotten entirely.
- Dirty air filter. A restricted filter richens the mixture and can cause hunting under load.
- Governor linkage or spring. A stretched governor spring or a linkage knocked during transport changes the response curve. Adjustment is machine-specific — follow the manual rather than guessing.
- Surging only with no load. Some engines hunt slightly at no load and settle immediately once 300 or 400 watts are drawn. That is normal behavior, not a fault.
Preventing all of this
Nearly every fault above is a storage fault, which means it is preventable for the price of a bottle of stabilizer.
- Add fuel stabilizer to every tank the machine will not burn within a month, and run the engine a few minutes afterward so treated fuel reaches the carburetor.
- Or run it dry. Close the fuel valve with the engine running and let it consume the carburetor bowl. This is what the fuel shut-off valve on modern generators exists for.
- Or store it on propane if it is dual fuel. Propane does not degrade, which is the strongest practical argument for dual fuel on a machine that mostly sits.
- Exercise it quarterly. Twenty to thirty minutes under a real load — a couple of work lights or a heater — every three months. This finds problems in your driveway rather than in a blackout.
- Change the oil on schedule. See how often to change generator oil; the first change on a new engine comes far sooner than most owners expect.
Two safety points while you work
Test-run the generator outdoors only, at least 20 feet from the house, with the exhaust pointed away from doors, windows and vents — never in a garage, a shed, or under a carport, even with the door open. The US Consumer Product Safety Commission states that a single portable generator can emit as much carbon monoxide as hundreds of cars, and links more than 100 US deaths a year to generator CO. It is colorless and odorless, so there is no warning. Fit CO alarms on every level of the home. Full detail: generator carbon monoxide safety.
And whatever the fault, never test a generator by plugging it into a wall outlet. Backfeeding energizes the utility line and can electrocute the line crews restoring power. A transfer switch or interlock kit installed by a licensed electrician is the only correct connection method.
Where to go next
If the machine turns out to be beyond economic repair, our ranked best portable generators and best quiet generators compare current models on aggregated owner ratings, including how often starting complaints recur by brand. For expectations on service life, see how long generators last. Everything sits under the power tools and workshop hub.
Sources: US Consumer Product Safety Commission Carbon Monoxide Information Center; Electrical Safety Foundation International, portable generator safety.
We never buy or physically handle the equipment we rank — our recommendations come from aggregated owner reviews and expert consensus, and the method is on how we rank.