Here is the sentence that sorts almost every generator problem in about ten seconds, and almost no troubleshooting page leads with it: a portable generator is two machines bolted together — an engine that makes the spinning, and an alternator (the "generator head") that turns that spinning into electricity. They are as separate as the motor and the alternator in your car. And they fail for completely unrelated reasons.
So before you touch anything, answer one question: does the engine run, or not?
- It won't start, or starts and dies → the problem is on the engine side, and it is almost always the fuel. The alternator is irrelevant — the engine never got far enough to spin it.
- It starts and runs fine, but the outlets are dead → the engine is healthy; the problem is on the alternator side, and it is almost always a tripped breaker or lost magnetism — not a burned-out generator head.
People lump both symptoms under "the generator is broken," and that phrase is how a working machine ends up at the curb at midnight. Once you know which side failed, the fix is usually cheap and often free. Let's take them one at a time.
Engine side: "it won't start" is almost always the fuel, not the engine
Here is the reframe that saves the most machines: gasoline is perishable. It is the part that died — not the engine. Most people treat gas as an inert, permanent thing sitting in the tank waiting to be used. It isn't. Pump gasoline in the United States is typically E10 — up to 10% ethanol — and it goes off in a way that is uniquely bad for a small engine that sat.
Two things happen while your generator sits full of fuel. First, the gasoline oxidizes and forms varnish and gum — a sticky residue. Second, the ethanol in it attracts and absorbs water out of the air. Both of those are murder on a small engine, because the whole thing hinges on a carburetor: a little brass chamber with passages and a main jet so narrow it's barely wider than a pin. Stale, gummy fuel dries into those passages and plugs them. Now the tank is full, the engine cranks, and no clean fuel can get through the varnished jet. The engine is fine. The fuel path is glued shut.
This is not a fringe cause — it is the cause. Small-engine makers are blunt about it: fuel can start to degrade in as little as 30 days, and gas that's more than a month or two old should be drained and replaced rather than run. The single most common reason a stored generator won't start is that it was stored with fuel in it and left to varnish the carburetor.
The cheap-first order for a no-start
Work down this list before you assume anything is broken. The first several cost nothing.
- Fuel valve OPEN. The petcock/fuel valve has to be set to open to let gas reach the carburetor. It's the most embarrassing no-start and one of the most common. Check it first.
- Choke CLOSED for a cold start. A cold engine needs the choke closed to start, then moved to open/run once it warms up. A warm engine started with the choke closed will flood. Get the choke position right before you crank repeatedly.
- Check the oil — really. Nearly every modern generator has a low-oil shutdown sensor that will refuse to start (or start and die in a couple of seconds) if the oil is low. This one masquerades perfectly as a fuel problem. Check the level on flat ground. Note the twist: sitting or running on a slope can trip the low-oil sensor even when there's plenty of oil, because the sensor reads the oil pooling away from it. Level the machine and try again.
- The fuel itself. If it's been sitting for months, the fuel is the prime suspect regardless of what else you find. Drain the old gas from the tank and — this is the part people skip — from the carburetor bowl too.
- The carburetor bowl and main jet. Here's the move that revives most "dead" stored generators: shut off the fuel valve, drain (or remove) the float bowl at the bottom of the carburetor, and clear the main jet — the little brass nozzle in the center stem — with a strand of wire or a needle, plus carburetor cleaner. This is the actual fix behind most of the machines a shop "brings back to life."
- Air filter and spark plug. A clogged air filter starves the engine of air; a fouled spark plug won't light the mix. Both are quick to check, clean, or replace, and both are cheap.
Alternator side: "it runs but there's no power" is the fix people never try
This is the one that gets working generators thrown away, and it's the most satisfying to fix because the answer is so often free. The engine roars, everything sounds perfect, and every outlet is dead. The owner concludes the "generator" — meaning the alternator — has burned out, and goes shopping. In most cases nothing is burned out at all.
Two causes account for the large majority of no-output complaints, and you check them in this order:
1. A tripped circuit breaker or GFCI (check this first — it's free)
Every generator has one or more output breakers, and many have a GFCI (the outlet with the little reset button). A surge, a fault in whatever you had plugged in, or just transport vibration can trip them, and a tripped breaker produces exactly the symptom "runs fine, no power." Push every reset button, flip every breaker off and back on, and try the outlets with nothing else plugged in. It takes ten seconds and it's the single most overlooked fix on the whole machine.
2. Lost residual magnetism — and how to "flash the field"
This is the genuinely non-obvious one. A generator makes electricity by spinning a magnetic field past its windings — but the magnetism it uses to get started isn't from a permanent magnet. It's a faint leftover charge in the iron called residual magnetism, held over from the last time the machine ran. It's what bootstraps the whole process at startup.
That residual magnetism slowly fades when a generator sits unused for a long time (and can be lost by storing it with a load plugged in, or running it a long time with no load). When it fades below a threshold, the alternator has nothing to build on: the engine spins the field, but there's no seed magnetism to amplify, so the outlets read zero. The generator isn't broken — it's forgotten how to start generating, and it needs to be reminded. Reminding it is called flashing the field, and there are two common ways:
- The corded-drill method (easiest). Plug an ordinary corded electric drill into the running generator, hold the trigger down, and spin the chuck backwards by hand. A drill is also a little motor; spun in reverse it acts as a generator and pushes a trickle of current back into the machine, re-seeding the magnetism. You'll often feel it "catch" and the outlets come alive. (A variable-speed drill works best.)
- The 12-volt method. Briefly touching a 12V source (like a car battery) to the field winding through the brushes will also re-magnetize it. This one involves opening the alternator and working near live wiring, so it's the method to hand to someone comfortable with it.
Only after the breaker and the field come up empty do you get into the genuinely broken parts: worn brushes, a failed automatic voltage regulator (AVR), or a bad capacitor on capacitor-excited units. Those are real, but they are further down the list than most people assume — and they're individual parts, not a reason to replace the machine.
What actually "died" — and why the repair is so cheap
Walk into any small-engine repair shop after storm season and you'll see a row of generators tagged "won't start." A large share of them will go home running after the same twenty-minute job: drain the stale fuel, clean the carburetor, clear the jet, fresh gas. The carburetor is one of the cheapest components on the entire machine — and cleaning it, or dropping in a replacement, costs a fraction of what a new generator does. The no-output machines are often even cheaper: a reset button, or a field flash with a drill you already own.
That's the whole argument of this page. The generator is a simple, rugged engine attached to a simple, rugged alternator, and neither one dies easily. What dies is the tank of gasoline, and what fades is a bit of magnetism. Both are cheap. Both are usually reversible in an afternoon. The expensive mistake is diagnosing "the generator is dead" and buying a new one to fix a problem the old one never really had.
Read your symptom
| What it's doing | Most likely cause | First move | New generator? |
|---|---|---|---|
| Cranks but won't fire; sat for months with fuel in it | Stale fuel varnished the carburetor | Drain tank & bowl, clean carb, clear the main jet, fresh gas | No |
| Won't crank / no fuel reaching engine at all | Fuel valve closed, or empty/water-fouled tank | Open the fuel valve; confirm fresh fuel | No |
| Fires, then dies after a second or two | Low-oil shutdown (or choke left closed on a warm engine) | Check oil on level ground; set choke correctly | No |
| Starts and dies on a slope but fine on flat ground | Low-oil sensor tripping though oil is adequate | Level the machine before starting | No |
| Runs perfectly, every outlet dead | Tripped breaker / GFCI (check first) | Reset every breaker and GFCI button; unplug all loads | No |
| Runs perfectly, no power, breakers are fine, sat a long time | Lost residual magnetism | Flash the field (corded-drill method) | No |
| Still no output after breaker reset & field flash | Worn brushes, AVR, or capacitor | Test/replace the individual part | Usually a part, not the machine |
When it really is a repair — or a replacement
Fuel and magnetism explain the overwhelming majority of dead generators, but not every one. Treat it as a genuine fault when:
- The engine has a mechanical failure. No compression, a knock, a seized recoil, or an engine that ran out of oil and was run anyway can mean real internal damage. That's an engine repair — and on a good machine it can still be worth it, but it's a different job than a carburetor clean. (The small-engine fundamentals overlap heavily with mowers, blowers, and pumps — see small-engine repair.)
- The alternator end is truly faulty. If the breaker's fine, the field's been flashed, and it still won't make power, you're into brushes, the AVR, or a capacitor — real parts to test and replace, but rarely a reason to scrap the unit.
- It's simply the wrong tool, or worn out overall. If a generator can't handle your critical loads, or it's an old workhorse that needs the engine and the alternator end done at once, replacement can be the honest answer. Our home generator buying guide covers running vs starting watts, inverter vs conventional power quality, and dual-fuel — and why a propane or dual-fuel unit sidesteps the stale-fuel problem entirely, since propane stores indefinitely. For small, quiet backup with no fuel to go stale at all, a battery portable power station is worth a look.