A generator that starts then dies usually points to fuel, air, spark, or sensor trouble—check the carb, cap vent, filters, oil, and load.
Power’s out, lights are dim, and your set fires up only to quit a minute later. This guide lays out clear steps, quick tests, and fixes you can do with tools.
Safety First So You Can Troubleshoot
Move the unit outdoors, at least 20 feet from doors and windows. Let the engine cool. Unplug loads before any testing. Wear eye protection.
Never run a generator in a garage, porch, or doorway. Carbon monoxide is deadly. Read the CDC generator safety facts and keep a CO alarm indoors.
For refueling, shut down and cool for 15–20 minutes. Wipe spills and store fuel in an approved can away from flames.
Why A Generator Won’t Keep Running: Rapid Checks
Start here. These checks solve most “starts then dies” calls:
- Fuel age and quality: If the unit sat 30–60 days, drain the tank and carb bowl, then add fresh gas. Water or stale fuel starves the engine.
- Fuel cap vent test: A clogged vent creates vacuum and the engine fades. Loosen the cap slightly; if it keeps running, replace the cap.
- Choke and air: A cold start wants partial choke, then off within a minute. A soaked or clogged air filter will stall the motor.
- Spark and boot: A fouled plug misfires under load. Remove, inspect, set the gap, and push the boot on until it clicks.
- Low-oil shutdown: Level the set and fill to the mark. Many models stop when oil is low or sloshes at an angle.
- CO shutoff: Newer models cut the engine when exhaust drifts toward people. Move the unit outside with the exhaust pointed away and retry.
- Overload or short: If it dies only with appliances connected, test with a single light and a known-good cord. Reset any GFCI outlets.
- Carb pilot jet: Runs only on choke or surges? The tiny pilot jet is likely gummed. Clean the carburetor or replace the jet.
- Altitude setting: If your model has an altitude or eco switch, set it for your location and load.
- Exhaust block: A clogged spark arrestor or a nest in the muffler will choke the engine.
Quick Symptom Guide
| Symptom | Likely Cause | 60-Second Test |
|---|---|---|
| Starts, then fades in 1–3 minutes | Tank cap vent blocked | Run with cap cracked open |
| Runs only on choke | Gummed pilot jet or vacuum leak | Spray carb throat; if speed changes, clean the carb |
| Stalls when load is added | Overload, bad cord, or tripped GFCI | Plug in one lamp; reset GFCI; try a new cord |
| Random shutdowns outdoors | Low oil, loose boot, or failing coil | Top off oil; press boot tight; test spark inline |
| Stops near a wall or door | CO sensor trigger | Move 20+ feet from openings; aim exhaust away |
Fuel System: From Tank Cap To Carb
Stale gas leaves varnish that shrinks fuel flow to a trickle. Ethanol blends pull in moisture and can phase-separate. If the machine sat, drain the tank and bowl, then feed fresh fuel with stabilizer. A clogged jet usually needs service. Water drops at the bottom of a clear fuel line or bowl are a tell. Rust flakes in the tank call for a rinse and a new filter.
Next, look at the cap. A tiny vent lets air replace the fuel you burn. When the vent clogs, the pump or gravity feed can’t keep up. The engine starts strong, then sags and stalls. Crack the cap. If it steadies and stays on, swap the cap and keep the old one as a spare.
Filters and lines matter too. A paper filter that looks tan or dark is done. Replace it with the arrow toward the carb. Soft lines that kink near the fuel shutoff will starve the bowl. Route lines with smooth bends and snug clamps.
Air, Spark, And Sensors
A clean filter and the right choke position keep the mix in range. Pull the element. If it’s soaked with fuel or caked with dust, the engine will bog or quit. Foam elements wash in soapy water and get a light oiling. Paper elements get replaced.
Spark comes next. Remove the plug. If it’s black and fluffy, you’re rich; if it’s white and blistered, you’re lean or too hot. Gap to spec and use the plug type the maker lists. Push the boot on firmly until it clicks. Weak spark under heat can point to a failing coil; an inline tester makes this a quick yes-no check.
Low-oil shutdown saves engines, but it can trip when the unit sits on a slope or oil is near the low mark. Level the frame, fill to the upper line, and retest. If shutdown repeats with full oil, the sensor or its wire may be damaged and needs service.
Many portables ship with CO sensors that cut the engine when exhaust builds near a home or crowd. That saves lives. Place the generator outdoors in an open area and point the muffler away. Never try to bypass the sensor.
Load And Electrical Side
Some engines stall when the alternator sees a big inrush, such as a fridge or well pump. Start with eco-mode off, warm the engine, add loads one at a time, then turn eco back on if your model has that mode. If a GFCI outlet pops, look for wet cords, crushed plugs, or a bad appliance. A hot, buzzing cord is a red flag; retire it.
If the engine falters only under load, check the inverter or breaker panel. Reset any tripped breakers. If power sags below spec, the governor linkage may be sticky. Lube pivot points and confirm the spring is intact. On models with digital error codes, read the label under the panel and note the code.
Weather, Altitude, And Storage Variables
Cold fuel and air change how an engine breathes. In freezing weather, use the oil grade the maker lists for low temps. Let the unit warm a few minutes before plugging in heavy loads. In dusty seasons, change the air filter more often. At elevation, a rich jet will flood the mix and kill the engine at idle. Many kits include a leaner jet and a note on which range fits your town. Some inverter models include an altitude toggle; set it before you start.
Simple Fixes That Solve Most “Starts Then Dies” Cases
- Replace stale gas, drain the bowl, and clean the pilot jet
- Swap a clogged air filter and set choke off once warm
- Crack the gas cap to test the vent and install a new cap if the test works
- Top off oil on a level pad and check the low-oil lead
- Move the unit into open air so a CO sensor won’t trip
- Test loads one by one with a known-good cord
Maintenance Table: Keep It Running
| Task | When | Quick How-To |
|---|---|---|
| Fuel refresh and stabilizer | Every 30 days of storage | Drain tank and bowl, add fresh gas with stabilizer |
| Air filter service | Check each 25 hours; replace 100 hours or as needed | Wash foam; replace paper; reinstall the cover |
| Spark plug | Inspect 100 hours; replace 200 hours | Gap to spec; torque lightly; snap boot fully on |
| Oil change | First 5–10 hours; then 50–100 hours | Warm engine; drain; new washer; fill to mark |
| Spark arrestor | 100 hours or dusty use | Remove screen; burn off carbon; reinstall |
| Carb clean | As needed when surging or choke-only running | Remove bowl and jet; spray passages; new gasket |
Parts And Tools To Keep On Hand
- One spare plug, a paper air filter, and a fuel cap
- Fuel stabilizer, carb cleaner, small funnel, and shop towels
- Inline spark tester and a basic multimeter
- New gaskets for bowl and jet, and a small assortment of clamps
Step-By-Step Carb Pilot Jet Clean
- Turn fuel off. Remove the bowl drain screw and catch the fuel. If water beads appear, go to a full drain and refuel.
- Remove the bowl. Unscrew the brass pilot jet from the tower. Hold it to the light; if you can’t see a round hole, it’s clogged.
- Soak the jet in carb cleaner, then blow through with compressed air. Never poke with steel wire; use nylon bristles or soft copper strand.
- Reassemble with a fresh bowl gasket. Turn fuel on and check for leaks. Start with half choke, then off. The engine should now idle on its own.
What If It Still Stalls?
You may be dealing with a sticking float needle, a failing ignition coil that quits when hot, or a governor issue. Coils often pass a cold test and fail under heat. If spark dies as the engine warms, replace the coil. A float needle that leaks will flood the intake and stall the motor; a fuel-shutoff test helps confirm this. Turn the valve off with the engine running; if it smooths out briefly before quitting, the carb was flooding.
Smart Setup That Prevents Repeat Stalls
- Park the set on a flat pad with the exhaust pointed away
- Use fresh E10 or less, or a non-ethanol fuel if available
- Add stabilizer at the pump, then label the can with the date
- Run the generator 20 minutes each month with a small load
- Log oil, filter, and plug service on a tag zip-tied to the frame
- Store a spare cap, plug, and filter in a zip bag taped under the handle
Linked Resources For Deeper Help
Manufacturer checklists and public guidance make troubleshooting safer and faster. See the Generac portable troubleshooting guide for step-by-step flowcharts and the CDC factsheet for placement rules.