Generator Won't Fire? Fuel, Spark and Carburetor Fixes
The power goes out and you need your generator. You pull the cord. Nothing. You pull again. Still nothing. Now what?
Generator starting problems almost always come down to three things: fuel that’s gone bad from sitting, a spark plug that isn’t firing, or the low oil sensor shutting things down. Below, certified mechanic Tom Bradley walks through each possibility step by step so you can figure out what’s wrong and get your generator running.
Quick Checklist
Run through this quick checklist first. About half of “won’t start” problems are actually operator error.
- Is there fuel in the tank? Sounds obvious, but check anyway.
- Is the fuel valve turned ON? Many generators have a petcock valve on the fuel line.
- Is the choke in the right position? Cold engines need choke closed. Warm engines need choke open.
- Is the engine switch set to RUN? Some generators have an on/off switch separate from the choke.
- Are the circuit breakers OFF? Generators start easier with no load connected.
- Is the oil level correct? Low oil triggers a safety shutoff on most generators.
If everything checks out and it still won’t start, keep reading.
Old Fuel Problems
Here’s the thing about generators—they sit in garages and sheds for months at a time, waiting for the next storm. That fuel sitting in the tank and carburetor goes bad faster than you’d expect.
How Fuel Goes Bad
Gasoline starts degrading in about 30 days. By 90 days, it’s causing problems. The lighter compounds evaporate, leaving behind a gummy varnish that clogs the tiny passages in your carburetor. That’s why a generator that ran fine last year won’t start this year even though you didn’t change anything.
How to Fix It
- Drain all the old fuel from the tank into a container for proper disposal
- Locate the carburetor bowl drain screw (usually at the very bottom of the carb) and open it to drain the bowl
- Add fresh gasoline—premium if available since it contains less ethanol
- Try starting with proper choke position
If the generator fires up after fresh fuel, you’re done. Run it for 15 to 20 minutes to burn any remaining old fuel out of the system.
If Fresh Fuel Doesn’t Work
The carburetor is probably clogged with varnish deposits. You have two options:
Option 1: Remove and clean the carburetor yourself. This involves taking it apart, soaking the body in carburetor cleaner, and clearing all the jets and passages with compressed air. Our carburetor cleaning guide walks through the process. It’s not hard, but it takes patience.
Option 2: Take it to a small engine shop. A carburetor cleaning runs $50 to $100 at most places and usually takes a day or two.
Spark Problems and Fixes
No spark means no combustion, no matter how fresh your fuel is. Here’s how to test.
The Spark Test
- Remove the spark plug using a spark plug socket (usually 13/16” or 5/8”)
- Reconnect the spark plug wire to the plug
- Hold the plug threads firmly against a bare metal surface on the engine block
- Pull the starter cord while watching the electrode gap
What you want to see: A bright blue spark jumping across the gap with every pull. This tells you the ignition system is working. Use a voltage tester or multimeter for precise measurements.
What you don’t want to see: No spark at all, or a weak orange/yellow spark. This means something in the ignition system has failed.
If There’s No Spark
Replace the spark plug first. It’s the cheapest and easiest fix. Generator spark plugs cost $4 to $8 and take two minutes to swap. A new plug solves the problem more often than you’d think.
Check the kill switch. A faulty kill switch can ground the ignition and prevent spark even with a good plug. Disconnect the kill switch wire from the ignition coil and try again. If it sparks now, the switch is bad.
Test the ignition coil. If a new plug doesn’t help and the kill switch isn’t the problem, the ignition coil has probably failed. Coils fail from age and heat cycling. Replacement coils run $20 to $50 depending on your generator brand.
Low Oil Shutoff Problems
Most modern generators have a low oil sensor that prevents starting when oil is low. It’s a safety feature that protects the engine from damage. But sometimes it causes starting problems even when oil is fine.
How the Sensor Works
A float sensor in the crankcase monitors oil level. When oil drops below the minimum, the sensor grounds the ignition circuit—same effect as turning the kill switch off. The engine won’t fire no matter how many times you pull the cord.
Checking and Fixing It
- Place the generator on a level surface (not on a slope or uneven ground)
- Remove the oil fill cap and check the dipstick
- Add oil if needed—most generators take about 0.5 to 0.6 quarts of SAE 10W-30
- Try starting again
If oil level is correct but it still won’t start: The sensor itself might be faulty. You can test this by temporarily disconnecting the sensor wire (usually a single wire coming from the crankcase area). If the generator starts with the sensor disconnected, replace the sensor.
Important: Don’t run the generator with the oil sensor disconnected. It exists to prevent catastrophic engine damage. Once you’ve confirmed the sensor is the problem, reconnect it and order a replacement.
Electric Start Problems
Many generators offer push-button electric start alongside the traditional pull cord. When the electric start stops working, here’s what to check.
Dead or Weak Battery
Generator batteries disc >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>??????????????????????????????????????? X For long-term storage, you have two good options: x Option 1: Run the generator dry. Let it run until it dies from fuel starvation, then close the fuel valve and drain the carburetor bowl. No fuel sitting means no varnish buildup.
Option 2: Keep treated fuel in it and run monthly. This works if you might need the generator on short notice.
Maintain the Battery
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Tools Needed
Tools:
- Spark plug socket (13/16” or 5/8”)
- Ratchet wrench
- Flathead screwdriver (for carburetor bowl drain)
- Multimeter (for advanced ignition testing)
Materials:
- Fresh gasoline
- Fuel stabilizer
- SAE 10W-30 motor oil
- Replacement spark plug ($4 to $8)
- Carburetor cleaner spray
Summary
When a generator won’t start, work through the problems systematically. Check fuel freshness first since that’s the cause most of the time. Test for spark if fresh fuel doesn’t help. Verify oil level to rule out the low oil shutoff. For electric start models, check the battery before assuming worse problems.
The best approach is prevention. Run your generator monthly, use fuel stabilizer, and keep the battery maintained. A generator that gets regular attention starts when you need it most.