It is the middle of an outage and the pull cord is doing nothing. Or you yanked the generator out for hurricane season after eleven months in the shed and it just sits there, dead. I have seen this exact scenario a thousand times — on aircraft fuel systems in the Air Force, on commercial equipment in supermarket back rooms, and on every neighbor’s generator that got dragged over to my garage. The mechanics are almost always the same, and the fix is almost always cheaper than people think.
The 30-second answer: If your generator won’t start and it has been sitting for more than a couple of months, the carburetor is the suspect about two times out of three. Drain the old fuel, drop a few drops of fresh gas straight into the spark plug hole, and try to start it. If it fires for a second or two and dies, the carb is gummed — that is the next thing to fix. If it does not even fire, move to spark and fuel valve next. Most fixes here are under $20 in parts.
The diagnostic order I use, in plain language
I do not start by tearing anything apart. I run three free checks first, because they tell me which of the nine causes below to actually go after. A generator needs three things to start: clean fuel reaching the cylinder, a strong spark at the right time, and decent compression. If any of those is missing, it will not run — and the order I check them in is the order that catches 90% of no-starts within five minutes.
Free check 1 — Fuel valve and tank. Fuel valve open. Gas in the tank that is less than 30 days old. Tank vent (the little hole in the cap) not plugged. I take the cap off and listen for a small hiss as it equalizes; no hiss either way is fine, but a strong vacuum-suck means the vent is plugged and the carb is being starved.
Free check 2 — Spark. I pull the plug, clip the boot back on, ground the threads against the engine block, and have someone pull the cord (or hit the electric start) while I watch the gap. A strong blue snap means I have spark. Weak orange or no spark means I am chasing ignition, not fuel.
Free check 3 — Choke and primer. Choke fully closed on a cold start. Primer bulb (where fitted) gives 3-5 firm pumps until it feels solid. If the engine fires for one second on choke then dies, that is a carb signature, not a bigger problem.
With those three checks done, I know which of the causes below is mine. Here they are in the order I actually find them, most-common first.
IMAGE_NEEDED: Photo of generator with the air filter cover off, carburetor visible, captioned “Most no-starts start here — the carburetor.”
Cause #1 — Clogged carburetor (the one you are probably looking for)
This is the cause behind roughly 60-70% of every “generator won’t start” call I see on stored portable units, and it is the one I want you to confirm or rule out before you do anything else. Here is why it dominates the failure list, exactly how to confirm it is your problem, and the three ways to fix it depending on how bad the carb is.
Why the carb is the #1 suspect on stored generators
Gasoline starts to break down chemically in about 30 days. Today’s pump gas is also blended with up to 10% ethanol, and ethanol does two things that destroy small-engine carbs: it pulls moisture out of the air into the fuel, and it leaves behind a gummy varnish when it evaporates. A portable generator that has been sitting for six months in a garage has carb passages that are basically painted shut with sticky residue. The main jet is the size of a sewing-needle hole, and even a thin film of varnish closes it off enough that the engine cannot draw the right air-fuel mixture to fire.
I learned this lesson on F-16 fuel systems where contaminated fuel could end an aircraft. Generators are not aircraft, but the chemistry is the same — fuel sitting still in a confined space turns into a problem.
How to confirm the carb is your no-start
This test takes 60 seconds. I take off the spark plug, drop 3 or 4 drops of fresh gasoline directly into the cylinder, screw the plug back in, choke closed, and try to start. If the engine fires for 2-3 seconds and dies, the cylinder is firing fine — fuel is just not getting there through the carb. That is a clogged carb signature. If it does not fire at all even with fuel in the cylinder, I move on to ignition (cause #3).
The three-tier fix, cheapest first
Tier 1 — Drain and refill ($0 in parts, 10 minutes). Sometimes the carb is not actually gummed, the fuel is just bad. I close the fuel valve, remove the float bowl drain screw at the bottom of the carb (10 mm or a flat-head depending on brand), and drain everything into a jar. If what comes out is brown, smells like turpentine, or has visible varnish flakes, the carb itself probably needs cleaning regardless. But sometimes draining the old fuel and refilling with fresh gas plus a shot of starting fluid in the air intake is enough to wake it up. About 15-20% of stored generators come back on this alone.
Tier 2 — Clean ($10-20 in parts, 60-90 minutes). Buy a carb rebuild kit specific to your generator’s carb model (the kit gives you gaskets and the float-bowl O-ring, which always tears when you take the bowl off). Pull the carb. Spray every passage with carb cleaner and follow up with compressed air. Pay special attention to the main jet — that is the brass nozzle inside the central stem of the bowl mount. A piece of stranded copper wire works to clear it if cleaner alone does not. Reassemble with new gaskets. This works on roughly 70% of dirty carbs.
Tier 3 — Replace ($15-45 for the carb). If the carb has been sitting wet for over a year, or if cleaning did not bring it back, a complete replacement carburetor is cheaper than my time. Aftermarket carbs for common Predator, Champion, and Generac portables run $15-45 on Amazon. Match the bolt pattern and the throat diameter from your old one, transfer over any throttle and choke linkage, and you are done in 45 minutes.
| Part | DIY price | Where | Time |
|---|---|---|---|
| Carb cleaner spray (Berryman B-12 or Gumout) | $5-8 | Auto parts store | — |
| Carb rebuild kit (model-specific) | $10-20 | Amazon / OEM | 60-90 min |
| Replacement carburetor (portable, generic) | $15-45 | Amazon | 45-60 min |
| Fuel stabilizer (next-time prevention) | $8-12 | Amazon / hardware store | 30 sec |
For the full walkthrough including which carbs interchange between brands, see the carburetor repair guides. If your generator fires for a few seconds then dies as soon as you let off the choke, that is the same problem in a slightly different form — covered in generator starts then dies.
Cause #2 — Stale or contaminated fuel (the upstream root)
Cause #1 is the carb. Cause #2 is the reason the carb got fouled in the first place. If I clean a carb but pour the same six-month-old gas back into the tank, I am going to be cleaning it again in two weeks. Drain everything from the tank, all the way down, and dispose of it properly. Refill with fresh gas from a station that turns over fuel quickly (avoid the marina gas at the end of the dock that sat all winter). Ethanol-free gas if you can get it locally — runs about 50 cents more per gallon and is worth it for stored equipment. See the fuel system guides for tank drain procedure and water-in-fuel diagnosis.
Cause #3 — No spark
If my free Spark check above came up dead or weak, I have an ignition problem, not a fuel problem. The cheapest place to start is the spark plug itself. I pull it, read it, and replace it if there is any doubt — a new plug is $4-10. A black, oily, or wet plug means the cylinder is being flooded with fuel that is not igniting (which usually means the plug is the culprit). A bone-dry plug with a tan-gray electrode means the cylinder is starving for fuel — go back to causes 1 and 2. See the spark plug guides for gap settings by brand and the difference between hot and cold plugs.
If a new, properly gapped plug still shows no spark when I test it grounded against the block, the next step is the ignition coil (covered in cause #7). On standby Generac units a no-spark condition often triggers an error code on the controller — those are covered separately in the Generac brand guides.
Cause #4 — Closed fuel valve or vapor-locked tank vent
This is the embarrassing one. I once spent 20 minutes on a generator before realizing the previous owner had left the fuel valve closed for storage. Always start with the obvious — valve open. The other version of this is a plugged tank vent. The cap has a tiny vent hole; if a wasp or dirt has plugged it, fuel cannot flow because vacuum builds in the tank. I run the engine briefly with the cap loose; if it suddenly runs fine, my vent is the problem and I clean the cap with carb spray and a piece of wire.
Cause #5 — Low-oil sensor false trip
Every modern generator has a low-oil shutoff sensor that kills the engine (or refuses to let it start) if oil level drops. They false-trip for two reasons: the generator is sitting on a slight slope so the float sensor reads low, or the sensor itself has failed. Level the generator, top off the oil to the upper line, and try again. If it still will not start with oil clearly full, the sensor is bad — about $10-25 to replace. Full walkthrough in low-oil shutdown.
Cause #6 — Dead battery (electric start units)
If your generator has electric start and the solenoid clicks but the engine does not crank, the battery is almost certainly the problem on a unit that has been sitting. Small generator batteries (12V 7-9 Ah, similar to a riding mower battery) have a useful life of about 3-4 years and die fast when left uncharged through a hot summer. I jump-start mine to confirm; if it cranks with jumper cables, the battery is the issue. Replacement is $30-70.
This is also why every standby generator should be on a trickle charger. A dead battery is the single most common cause of a Generac home standby failing to start in AUTO mode during an outage.
Cause #7 — Failed ignition coil
If you replaced the spark plug, gapped it correctly, and still have no spark, the ignition coil is the next suspect. The coil sits next to the flywheel and fires the plug each revolution. They are reliable parts that do not go bad often, but heat-cycled coils on older units (10+ years) do eventually fail. Test resistance with a multimeter against your manual’s spec — typical primary side reads 0.5-2 ohms, secondary side 5-10 kilo-ohms. Coils run $15-40 to replace.
Cause #8 — Air filter (rarely the cause, but free to check)
A completely plugged air filter will not stop a generator from starting — it will just run rich and smoke. But a filter saturated with oil (which happens if the generator was tipped during transport) can absolutely cause a no-start because no air gets through. I pop the air filter cover, check the foam or paper element, clean or replace if needed. A new filter is $6-15. While I am in there, I check for rodent damage to the intake — mice love to nest in air boxes during storage and I have pulled out everything from acorns to insulation pieces.
IMAGE_NEEDED: Side-by-side photo of two spark plugs — one healthy tan-gray, one oily black, captioned “Reading the plug. The one on the left is happy. The one on the right is telling you the carb is dumping fuel.”
Cause #9 — Mechanical failure (compression, valves, sheared key)
If I have walked through all eight causes above and still have a generator that will not start, I am into mechanical territory and the math starts to favor replacement on a small portable. Low compression from worn rings, a stuck valve, or — on units that backfired hard — a sheared flywheel key. A compression tester ($25 at a parts store) tells me if compression is there; under 60 psi cranking on a small generator engine is a problem. Valve adjustment is a specific procedure per engine — if you are reading this article you are probably not at that point yet.
Safety — read this before you turn anything.
Before working on the engine of any generator, pull the spark plug wire off the plug and tuck it where it cannot snap back. A small engine can kickback hard enough to break your wrist if it fires with your hand on the cord. Never test a running generator indoors or in an attached garage — carbon monoxide is invisible and kills people every storm season. Generator exhaust outside, at least 20 feet from any door, window, or vent, period. And if you are testing the unit by connecting it to your house, never backfeed through a wall outlet — that is illegal and can kill a utility lineman on the grid. Use a transfer switch or interlock. See generator safety for the full breakdown.
When DIY makes sense and when it does not.
A small-engine shop will charge $75-150 just to look at the generator, plus $90-160 for a carb clean and rebuild, plus parts. For a unit you paid $400-700 for new, two service trips and you are at half the cost of a new generator. The DIY path on causes 1-6 above runs $20-50 in parts and a Saturday morning. The math flips when you have a $4,000+ home standby or a $2,000+ inverter — at that level, a factory-authorized dealer is worth the diagnostic fee because a misdiagnosis on the control board can cost real money.
How I prevent this happening next storm season
Most of the no-starts I diagnose were preventable. Three habits keep my generator (and everyone’s I have helped set up) starting reliably:
1. Fuel stabilizer in every tank. Sta-Bil or equivalent, dosed per the bottle directions, every time I fill the tank or the gas can the generator gets fueled from. Costs pennies per tank and pushes fuel stability from 30 days out to 9-12 months. This is the single biggest preventive lever.
2. Run the generator under load for 15-20 minutes once a month. Plug in a space heater or a couple of work lights as a load. This circulates fresh fuel through the carb and keeps the seals on the float bowl from drying out. Standby units do this automatically as their weekly exercise cycle — portables need you to do it manually.
3. Storage prep before any long sit. If the generator is going away for more than 30 days, I either run it dry (run with the fuel valve closed until the engine quits, which empties the carb bowl), or I drain the carb bowl manually via the drain screw. Either kills the stale-fuel-in-the-carb pattern that creates 60% of the no-starts above.
Full preventive maintenance schedule by month is in generator maintenance.
IMAGE_NEEDED: Photo of fuel stabilizer bottle next to a gas can, with a generator in the background, captioned “Sta-Bil in every gallon. Cheapest insurance policy I know of.”
Video walkthrough: carburetor cleaning step by step
If you have decided your carb is the problem, this walkthrough covers the actual disassembly and cleaning process on a typical portable carburetor. The steps look the same on Predator, Champion, and most Honda-clone engines.
Frequently asked questions
How long can gas sit in a generator before it goes bad?
About 30 days of stability for pump gas with ethanol, and roughly 9-12 months if you added fuel stabilizer when the tank was filled. Ethanol-free gas extends that to about 6 months unstabilized. After those windows, you are on borrowed time and the carb is starting to gum.
Can I run my generator on starting fluid?
Starting fluid is a diagnostic tool, not a fix. A short squirt into the air intake will tell me if the cylinder is firing — if it briefly runs on starting fluid then dies, I have confirmed the carb is starved. But repeated use is bad for the engine because ether ignites harder than gasoline and accelerates wear. Use it once to diagnose, not as a workaround.
How do I know if my carburetor is bad or just dirty?
Cleaning works on about 70% of dirty carbs. If I clean the carb thoroughly, replace the gaskets, and the engine still will not start cleanly or it surges badly, the carb body itself is probably warped, corroded internally, or has a damaged emulsion tube — replace it. Replacement is usually cheaper than a second cleaning attempt at that point.
What is the difference between a portable and a standby that will not start?
Portables fail mechanically — carb, plug, fuel. Standby home generators (Generac Guardian, Cummins, Kohler) fail electrically far more often. The single biggest cause of a standby failing to start in AUTO during an outage is a dead battery. Second is a stale-fuel issue if it has been months since the last exercise cycle. Standby fault codes also light up on the controller — see the Generac guides for code lookups.
My generator was running fine yesterday and now will not start — what changed?
Sudden no-starts that were not preceded by storage usually come down to spark plug, fuel valve left closed accidentally, or low-oil sensor. Recent runs that ended hot can also cause a vapor lock — let the unit cool for an hour and try again. A unit that was working fine and then suddenly will not turn over at all (no compression sensation in the pull cord) is the rare mechanical case in cause #9.
Is it worth fixing an old generator or should I just replace it?
If the engine has good compression and the no-start is in the fuel or electrical system, fix it — $15-50 in parts will outlast another decade. If compression is shot, the frame is rusted through, or the unit is 15+ years old with multiple problems stacking up, the math on a new 4,000-watt portable at $400-600 starts to make sense. Standby units are different — a control board on a 10-year-old Guardian can run $400+ and it is often worth it because the engine and alternator behind it are still good.
The pre-storm check (the version to remember)
- If the generator sat — start with the carburetor. Roughly two-thirds of stored no-starts trace back to gummed fuel passages.
- Three free checks before any tools: fuel valve open, fresh gas in the tank, strong blue spark at the plug. That order catches almost everything.
- The cheapest fixes — drain and refill, new $5 plug, new $10 cap or vent — solve more no-starts than any expensive part.
- Electric-start units: if the solenoid clicks but the engine does not crank, replace the battery first.
- Standby units that will not start in AUTO are usually a dead battery or a stale-fuel problem, not the alternator end.
- The cheapest insurance for next year: fuel stabilizer in every tank, monthly run under load, and store dry if you will not run it for 30+ days.
If none of the nine causes above matches what your generator is doing — for example it starts and then dies, or runs but puts out no power — try the Fix My Generator symptom selector for a different starting point. For brand-specific symptoms see the Generac, Predator, and Honda guides.
For deeper background on small-engine fuel chemistry and why ethanol blends are tough on stored equipment, Briggs & Stratton has a solid technical writeup that lines up with what I see in the field.