Generator Backfiring? Reasons and Quick Solution!

Why is My Generator Backfiring

At times, a generator can surprise us, and not in a good way. You try pushing the ignition key on or pulling the recoil to start the engine, and the engine starts, but the engine backfires and runs rough or shuts down. The next thing that comes to your mind is, why is my generator backfiring?

Your generator might be backfiring because of a lean air-fuel mix, miss-timed combustion, or some air stuck in the intake valve. A generator can also backfire because of a late explosion; air stuck in the exhaust valve, or rich air-fuel mixture, which mainly occurs as late backfiring. The backfiring can happen at the carburetor or exhaust, the most common.

And since you have experienced the backfiring problem, let’s go through how you can troubleshoot and solve it and get your generator running okay again.

Why is Your Generator Backfiring

A generator backfiring can happen either happen before reaching the ignition chamber or after the fuel ignition. I will take you through troubleshooting these two types of backfiring, and in the end, you’ll have an idea of what is happen when you hear the sound or see fire out of the carburetor or exhaust.

1. Carburetor Backfire

When the engine receives a lean air-fuel mixture, the intake valve gets stuck open, or there are some timing issues with the ignition, the backfiring will happen in the carburetor. Let’s digest each of these issues individually.

a) Reduced Fuel-Air Mixture

If you press the engine ignition key or pull the recoil start and the engine receives a lean air-to-fuel ratio, there will be slow ignition inside the combustion chamber.

During fuel combustion, the piston will be on the power stroke, but it’s slower than usual because of the improper air-fuel mixture ratio.

Because of this, the combustion process continues as the piston moves to the exhaust stroke to expel the burnt fumes. As the exhaust stroke nears the end, the intake opens and causes valve overlap.

There is new fuel entering the combustion chamber pull by the vacuum state created during the exhaust stroke during the intake.

As the new air fuel flows in, the slow combustion can easily ignite it instantly, and it can travel back up to the carburetor. That’s when you get the pop, a flame, or fire.

What might be the cause of the problem? The problem is mainly caused by restricted jets allowing little fuel to flow. There is also a vacuum leak in the generator gaskets between the combustion chamber and the air filter.

It can also be a misconfiguration of the carburetor – the tiny screws used for controlling the air getting through to the engine. There might also be dirt inside it that might be restricting air from getting through to the engine.

To solve the slight air-flow ratio problem, you have to inspect all these parts. Replace the damaged gaskets, clean the carburetor, and reconfigure it. Reopen the jets to allow free fuel flow.

b) Combustion-Intake Timing Issue

During the intake stroke, the engine piston goes downward, pulling in the air-fuel mixture. When the piston is moving back up for the compression stroke, the fuel valve should stay closed.

When the timing is off, the spark plug will produce the spark to ignite the air-fuel mixture compressed before the intake valve closes.

When the combustion occurs, the engine backfires to the carburetor, blowing back the air-fuel mixture out.

What might have caused the problem? You could be experiencing this problem if there is a shared flywheel key, faulty spark plug, or faulty ignition coil.

c) Stuck Fuel Intake Valve

I did touch on this while explaining the two prior problems. When the intake valve opens, it allows fresh air-fuel mixture into the combustion chamber. It’s supposed to close when the compression stroke starts its push. When it’s in the open position, it can cause a backfire on the carburetor.

Stuck Fuel Intake Valve

It’s possible if the rocker’s arm is loose, wrong clearance between the intake valve stem and the rocker, pushrod, or tappet. A decompressed spring can also be caused when one of its clips falls out or a rusty valve.

When the intake valve stays open, the combusting air-fuel mixture or the exhaust gas will blow back causing the backfire through the carburetor when it’s wide open or through the exhaust or both.

What might have caused the stuck fuel intake valve problem? To check for a faulty intake valve, you have to get into the valve head by removing the top valve cover. Check the tappet area or the rocker arm to make sure all the springs are compressed, and there are enough clearances; there should be nothing bent or damaged.

With the spark plug out, pull the generator recoil cord slowly, keeping your eyes on the valves, and the rocker’s arms are; they move; everything should be moving.

You can remove the combustion chamber head to diagnose the problem further. Check for damaged valve face and too much carbon build-up between the valve face and its seat. These two issues can prevent the intake valve from closing correctly.

Exhaust Backfire

Most people are more familiar with exhaust backfiring than carburetor backfiring. It’s the loud boom sound and possible flame you get when starting the generator or when it’s running. The cause of exhaust backfires mainly occurs after combustion.

a) Reduced Air-Fuel Mixture

Like with carburetor backfire, exhaust backfire can also happen when the generator engine runs on a lean air-fuel mixture. Since the mix will burn slowly, it continues to burn as it’s exhausted.

When the air-fuel mixture burns in the exhaust pipe, it creates a backfire. There will be some that go back to the carburetor, depending on how worse the problem is.

When you see flames coming out of the muffle, it’s caused by the combustion process underway when the fumes are exiting.

b) High Air-Fuel Mixture Ratio

If you asked a mechanic, the number one cause of exhaust generator backfiring is the engine receiving low-air-fuel mix in the combustion chamber.

If the fuel entering the chamber is higher than what the engine should be receiving concerning air, the oxygen level needed to ignite for the combustion to occur won’t be enough.

When the piston moves to the power stroke, some unburnt fuel will exit the hot exhaust during the exhaust stroke.

When the unburnt fuel fumes meet with oxygen in the hot exhaust manifold, it spontaneously combusts, making a banging backfire sound. Its intensity depends on the amount of unburnt fuel entering the hot exhaust pipe.

c) Delayed Combustion

As I mentioned before, early combustion causes a backfire through a carburetor, exceptionally when the combustion process starts when the intake valve is partially closed. The same can happen in the exhaust.

During the compression stroke, the spark plug is supposed to go off. If it’s delayed and produces sparks late when it’s descending by itself because of the moment, the combustion will happen late.

That allows the unburnt air-fuel mix to escape the combustion chamber and ignite in the exhaust manifold. If that were to happen, the generator would backfire, producing loud bangs or flames.

The delayed combustion can happen when the flywheel key is sheared, affecting the signal sent to the ignition coil to produce the spark. It can also occur when there is a problem with the spark plug or an issue with the ignition coil.

That’s why it’d be best always to have some spare spark plugs hanging around; they are pretty cheap pieces that can save you from opening your generator engine unnecessarily.

d) Stuck Exhaust Valve

When there is too much build-up of carbon on the valve seat or face, it’ll not close properly. It could also be because of damage to the valve or wrong clearance preventing the valve from providing the ideal seal. That means, during the compression stroke, the piston might push out some air-fuel mixture into the exhaust.

Diagnosing the problem requires you to remove the cover on the valve and check the rocker arms, spring compression, and clearances. Ensure all the valves mode, the clearances are alright, and the springs are correctly compressed. Do the same for an OHV engine, but remove the combustion chamber.

How Do You Fix a Backfiring Generator?

Generator backfiring can be continuous or a rare thing. If your generator backfires continuously, you may have a problem with the air-fuel mixture, most likely caused by a carburetor or fuel issue. It could also be a faulty spark plug. The best solutions for such issues can be one of the following:

a) Carburetor Cleaning

Cleaning your generator carburetor can help you solve the most common issues associated with a generator engine. According to my mechanic friends, a carburetor is a standard part of most small engine problems.

How to clean a Carburetor:

Step 1: Unplug the spark plug to prevent accidental engine start
Step 2: Locate the fuel valve on your generator with your generator manual’s help and set it off.
Step 3: Use the same generator manual to locate the carburetor.
Step 4: Disconnect all the hoses connecting to the carburetor, including the fuel line.
Step 5: Open the carburetor drain and drain any fuel that might be remaining.
Step 6: Get a spanner that can open the type and size of bolts used to hold the carburetor to the generator engine shell.
Step 7: Disassemble the carburetor, carefully placing the pieces somewhere the same to avoid losing them.
Step 8: Careful remove the main jet and set it aside. You can use soft jet brushes to clean the carbon inside the tiny jet hole.
Step 9: Get the Carb Cleaner and spray it into any hole you see in the carburetor housing. Try reaching inside and wiping or brushing the dirt out.
Step 10: Spray the same clean to the mean jet, primarily through the tiny hole.
Step 11: Reassemble everything back and connect everything as you found it.

b) Carburetor Configuration

If the carburetor is clean and you’re still getting the carburetor backfiring, there could be an issue with its configuration. The ratio of the air to fuel has to be suitable for the engine to burn it properly.

If that’s the case, you can get yourself a carburetor adjustment kit and try adjusting it if you know what you’re doing.

Else, the best option is to take the generator to a generator repair shop, and the technician can adjust it for you. I cannot recommend you temper with the adjustments as you might make the problem worse.

c) Spark Plug Replacement

Remove the old spark plug and check its condition. If the tip is covered with carbon, cleaning it might not work, or the problem might return almost instantly.

The best option is to get a spark plug replacement, a new clean piece. A set of these plugs won’t cost you much.

d) Clean the Valve or Rectify the Clearance Space

If your generator backfires when starting, the engine starts but shuts down or doesn’t start at all; the focus should be on the valve. The valve clearance between its stem and the rockers, pushrods, or tappets might not be enough.

The springs might be decompressed if it’s an OHV engine, there could be carbon build-up between the valve’s face and seat, or the rocker’s arm is bent.

You can grind the valve stem down a tiny piece to give it the proper clearance between it and the pushrods or tappet.

e) Shutdown Your Generator Properly

When the generator backfiring occurs when you turn off the engine abruptly when running at high RPM, the cause of the backfire could be caused by incomplete piston cycles without spark.

It will still suck in air and fuel into the combustion chamber if it cycles with the ignition system. If the muffler is hot enough, the fuel mixture can ignite and cause the loud bang to backfire.

You can prevent this, though, by shutting down the generator properly. Unplug the load and set the generator to idle for a few seconds before you can turn it off.

Can Generator Backfiring Damage the Engine?

The best answer is, it depends. Before you can judge that a generator backfire will damage your generator engine, there are a few things you might want to consider:

Soft, Non-Violent Backfire

Most times, the exhaust backfire, the popping sounds coming out of the exhaust; it’s not an issue for immediate damage if the sounds are low.

But because of the escape of an unburnt fuel-air mixture, the generator efficiency will reduce, and try to achieve it by overworking the engine. That way, there will be an increased risk of damage and wear in the long run.

Loud, Violent Backfire

If your generator backfires with a loud and violent bang sound that scares you even when you a distance away, the aftermath of the backfire has the chance of damaging your exhaust manifold and the pipe itself.

When the backfire happens through the carburetor, the risk even increases. Since the fire is happening in the direction of the fuel, there is an increased risk of fire. The small gas in the carburetor can combust, and before you know it, your generator is on fire.

So, please don’t allow your generator to run when it’s backfiring through the carburetor. Try troubleshooting using the above tips. If you’re not a handy person, take it to a technician for a checkup and repair.


Allowing the generator to run while backfiring is risky; it can cause fire damage, or you might be disturbing your residence. Besides, your generator might not even keep running or even start when there is a backfire. So, the best decision would be to troubleshoot and fix the problem as soon as possible.

Sharif Maih

Hi, I am Sharif. I would work in a generator manufacturing company. I worked there for a few years then I started my own business selling generator. But the reason for creating this blog site most of people do not know what type, size, kind of generator they should buy for their self. They do not have enough knowledge about generators which is why the choose the wrong generator. I think my blog can help them to choose the right Generator.

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