3 Reasons Why a Portable Generator Backfires

Normally a generator shouldn’t produce some pop sounds. Also, if you notice a thick smoke coming from the exhaust or carburetor, something about it is wrong. Often, most people conclude that those are signs of a backfire. Somehow, they are true. But there is a difference between a backfire and an afterfire. The former creates a pop sound or smoke from the carburetor. The latter happens when there’s smoke or pop sound from the exhaust. So what causes these issues? 

Generators often backfire due to issues with the air-fuel mixture, early combustion, or problems with the intake valve. An afterfire happens because of late combustion, exhaust valve issues, or air-fuel mixture problems. Diagnosing the cause of a backfire or afterfire is crucial to finding a remedy.

In this article, we are going to take you through the possible causes systematically. Take a look. 

What Causes Generators To Backfire?

A backfire is when there is a challenge in the generator carburetor. A single issue or a combination of several problems can cause your generator to backfire. The following are the leading causes of the issue:

  1. A limited mixture of air and fuel in the carburetor 
  2. Early combustion of fuel
  3. An issue with the intake valve 

1. A Limited Mixture of Air and Fuel in the Carburetor 

For efficient combustion to occur, there must be a sufficient supply of air to the fuel. If the supply of air is insufficient, the rate of fuel combustion will be slow. You’ll often find that even while the supply is slow, there is still combustion of fuel. But the rate is so slow that combusting a unit of fuel will take more time than usual. 

As a result, combustion will still be in progress when the power stroke comes to an end. Therefore, when the generator’s piston goes up in the exhaust stroke, it will be interrupting the combustion process. 

Two things happen in this step that causes a hitch in the generator. First, the exhaust valve opens, which is what you should ideally expect during the exhaust stroke. Second, the intake valve also opens to allow fresh air and fuel supply into the combustion chamber. 

Note that at this instance, there is continuing combustion of the previous air and fuel mixture. The flame of the slowly burning fuel will ignite the new incoming mixture, which will result in a sudden pop sound. Also, it can be associated with a flame which will also cause a backfire to occur. 

The primary remedy to such a challenge is to check the air jets. Probably, they are not allowing a sufficient supply of air into the combustion chambers. Also, there may be a gasket leak in the connection of the air filter and the combustion chamber. As a result, air that comes in leaves before it entirely burns. 

Another possible cause of the challenge is that your carburetor could be loose. In that case, it would allow air to exit through the sides, thus inhibiting proper combustion. Lastly, it is also possible that your generator is running on low-quality fuel. 

Many of the combustion problems in a generator are in the carburetor. Hence, it is a component that you should handle with caution. 

2. Early Combustion of Fuel

A backfire can also happen when there is early combustion in the carburetor. It results from a spark that comes earlier than usual. 

Typically, the piston presses downwards during the intake stroke to allow the entry of the air-fuel mixture into the combustion chamber. Once sufficient air and fuel have come in, the piston starts rising. In this phase, the intake valve should shut. 

However, in some cases, the valve fails to shut on time. If that happens and the spark plug produces a spark, there will be combustion of the air-fuel mixture on the combustion plug. The burning air and fuel will also move to the carburetor by the piston, thus causing a backfire. 

Several reasons can cause the fault in timing. First, it could be that the ignition coil is faulty and ignites at the wrong time, there could be a damaged spark plug, or there could be a problem with the flywheel key. 

3. An Issue with the Intake Valve

An intake valve is very crucial in the functioning of the fuel combustion system. It opens or closes systematically to allow the entry of the air-fuel mixture into the combustion chambers. When it opens at instances when it is supposed to be closed, expect a backfiring of the carburetor.

There can be several causes of a faulty inlet valve. First, it could be that the rocker arm area of the system is faulty. Also, the coordination of the pushrod system and the valve could be dysfunctional. It may also occur if the valve is worn out through tear and wear or rusting. 

In either of these problems, the intake valve will open at instances when it should be closed. As a result, it will allow the flow of fuel when it shouldn’t. The air-fuel mixture then goes back to the carburetor. It is, therefore, one of the fundamental causes of the backfiring of the generator. 

You can diagnose the issue by first taking off the cover to confirm that the rocker arm is okay. Also, you should confirm the position of the springs. Ideally, they should be in a compressed state. Therefore, if they are not, then there is a problem. 

Lastly, you should also check the clearances for any signs of bending. 

You can check this by yourself or involve a mechanic. It is pretty simple, though, to diagnose if there is an issue. Also, it would help to remove the spark plug and observe the motion of the rocker arms when you pull the recoil cord. If the valves are not shifting their positions, there is a problem that you need to fix. 

Other common problems of the valves include the accumulation of carbon between the face and the seat. If present, the build-up hinders the valves’ closure and prompts the movement of exhaust fumes backward to the carburetor. 

Other Reasons Why a Generator Backfires or Won’t Start

People commonly use generators during emergencies, such as when there is a power outage. Hence storing it for some time is almost inevitable. But you may find that it won’t start after storing it for some while, or it may produce some backfiring sound. 

We have already covered the common causes of a backfire or an afterfire. Now we’ll look at the additional causes of a backfire and reasons why your generator won’t start

Low on Oil

A generator will not start, or it will backfire if it’s low on oil. Therefore, it is imperative to check that the oil is at the appropriate level regularly. A dipstick will be handy in this process. 

However, you must know that placing the generator on an uneven surface will affect the oil flow. The generator could be having sufficient oil but, due to its orientation, fails to detect it. Thus, ensure that you always operate it on an even platform. 

Ran Out of Fuel

If your generator does not have sufficient fuel, it will probably not start. Also, you may experience some backfiring if you try to run it on low fuel. Therefore, it’s imperative to add gasoline when it’s low. Also, ensure that you use fresh fuel to avoid clogging the generator’s combustion system parts. 

Choke Lever is Wrongly Positioned

You must ensure that the fuel choke is in the correct position when starting the engine. Ideally, it should be in a closed position when you are preparing to start the engine. Once it has fully warmed up after some time, you need to change it to an open position. 

Closed Fuel Valve

Your fuel valve’s position could be the reason that the generator is not starting or is popping. The valve controls the movement of the fuel from the tank to the carburetor. Hence, it should be open when starting the generator, especially if you had drained the system during a shutdown. 

At times, you may find that the fuel valve is open, but the fuel is irregularly flowing to the carburetor. In such a case, you need to check the position of the vacuum relief valve. It should be open. Also, you can check if there is an issue with the valve by removing the fuel cord where it joins the valve. If no fuel is flowing and the valve is open, then it means that it is clogged. 

Another critical part that you should check is the fuel filter. Over time, it could become clogged, especially if you’re using low-quality gasoline. 

Issues with the Carburetor 

Your generator’s carburetor could be blocked, especially if you have stored it for a while. This is a common issue when storing your generator without draining all the fuel from the system. Fortunately, you can fix this easily. Open the carburetor drain while the fuel valve is in a closed position. 

Alternatively, remove the carburetor’s lower part and thoroughly clean it with clean gasoline to remove the sediments. 

Dysfunctional Spark Plug

Sometimes, your generator may fail to start because the spark plug is dysfunctional. You can remove it and replace it with another if you’re sure that it has a problem. Alternatively, you can clean it up by removing the carbon deposits on the part that joins the crankcase. 

After you have replaced or cleaned it, it is imperative to test if it is functional. First, connect the current supply to the spark plug. Next, pull the recoil starter like you do when starting the engine. At this instance, you should be holding the plug to the crankcase. If it is fully functional, it will produce a blue flame. If it doesn’t, consider checking if the ignition coil works. 

If the ignition coil is operational, the problem is the spark plug. 

The Battery Could be Dead

If your generator requires a battery to start, it’s essential to check if it’s working, especially if you had stored it for some time. It could be low on charge, and all you need is to recharge it. 

What Causes Generators to Afterfire?

Some people cannot differentiate between a backfire and an afterfire as they all sound the same. Both of them happen due to a fault in the generator’s combustion system. Thus, if you cannot differentiate between the two, it is because you probably cannot identify where the pop sound is exactly emanating from.

The following are the common causes of an afterfire issue:

  1. A limited supply of the air-fuel mixture
  2. An excess supply of the air-fuel mixture
  3. A delay in combustion
  4. An issue with the exhaust valve

1. A Limited Supply of the Air-Fuel Mixture

Like a backfire, you can also experience an afterfire resulting from the slow combustion of fuel. When there is a challenge in the combustion system, the rate of burning is slow. As a result, the burning air-fuel mixture can return to the carburetor during the compression phase resulting in a backfire. 

At the same time, the bulk of the burning air-fuel mixture can pass to the exhaust during the exhaust stroke. The result will be a release of flames from the generator’s exhaust, accompanied by a pop sound. The sound is a sign of abnormal combustion. 

Such an occurrence is called an afterfire, and it is similar to a backfire. However, it happens in the exhaust rather than in the carburetor, just like a backfire. 

2. A Rich Supply of the Air-Fuel Mixture

Sometimes an afterfire will occur as a result of a rich supply of an air-fuel mixture. If the air entering the combustion chambers is insufficient to fully aid the fuel’s combustion, some will be unburned. 

Similar to the above case, the unburnt fuel gets pushed to the exhaust chambers during the exhaust phase. The unburnt fuel will come into contact with a heavy supply of oxygen in the exhaust chambers, which will result in a rapid ignition. Also, note that the exhaust chamber is hot at this instance. 

From the interaction, there will be a loud pop resulting from the ignition of the fuel. The pop’s loudness will depend on the degree of unburnt fuel that contacts the air. 

3. Delayed Combustion

In the previous section, we explained that early combustion could cause a backfire. Similarly, there can be a delay in combustion in the chambers, which will cause an afterfire in the exhaust. 

An afterfire in the case of delayed combustion will happen in the piston’s compression phase. In this step, the spark plug may delay releasing the ignition spark. Resultantly, the air-fuel mixture will combust at a late stage of the combustion process. 

Rather than burning in the previous phase, it will now combust in the exhaust step. Hence, as the piston moves downwards during compression, it expels a burning air-fuel mixture instead of driving out burnt gases. When the mixture reaches the exhaust chamber, it ignites due to an ample supply of oxygen. Also, note that the exhaust fold is also extremely hot, and thus a rapid burning will happen to release a pop sound. 

The most probable cause of the poor timing of the spark plug could be the plug itself. It could be faulty and thus release sparks at irregular instances. Also, shear in the flywheel key will result in an inconsistent spark release. Lastly, the issue could be the ignition coil. 

Engage your mechanic to identify what the actual cause of the problem could be. 

4. An Issue With the Exhaust Valve

In the case of a backfire, we explained that an issue in the inlet valve could be the precursor to the issue. Similarly, a valve problem could cause an afterfire, but in this case, it’s the exhaust valve. 

Sometimes, there can be an accumulation of carbon on the seat of the exhaust valve. As a result, the valve will not perfectly seal the area. Therefore, without a proper seal, the unburnt air-fuel mixture will pass to the exhaust chamber. In the above cases, the mixture will ignite rapidly, causing a loud pop sound. 

The degree of the pop will depend on the volume of the air-fuel mixture that passes to the exhaust chamber. You can solve the issue by removing the carbon deposit on the seal area. 

How Do You Fix a Generator that Backfires?

You can solve most backfire issues, especially if you identify if you are dealing with a one-off or persistent problem. Also, you should diagnose if the issue happens after starting the engine, shutting it, or decelerating the engine. 

If the problem is recurrent, there is a significant likelihood of an issue with the air-fuel mixture. In that case, it is imperative to clean the carburetor efficiently. Also, check the valves for the presence of accumulated soot and remove it. Cleaning the carburetor will go a long way toward fixing the problem, as most backfire challenges stem from the component. 

An issue in the valves could cause a backfire that happens during the engine start. In such a case, check the condition of the pushrods and the springs. If the springs are decompressed, then fix them as they are possibly the cause of the problem. Also, check if there is an accumulation of carbon on the valves. 

You could also be experiencing a backfire when switching off the generator. In such a case, first, ensure that you unplug all the connected appliances to the generator. Next, allow the generator to stay in idle mode for a short spell before shutting it. The time will allow normal combustion, thus limiting the probability of a backfire. 

The other possible scenario is when the generator backfires when you suddenly shift to a low RPM. The cause of the backfire, in this case, is the sudden increase in the fuel that the generator requires to combust against a slowly moving engine. Hence, it is a normal thing that shouldn’t worry you. 

Can a Backfire or Afterfire Damage a Generator’s Engine?

We cannot straightforwardly tell if a backfire can damage a generator’s engine as it will depend on several factors. 

If your generator produces afterfires, it’s a big problem, especially if the sound is loud. Such an issue may probably affect the functioning of your generator. It will strain to function normally, which will contribute to wearing out in the long run. A worst-case scenario is when the afterfire is too loud. In such a case, it will damage the exhaust system after some time.

While you can afford to ignore an afterfire if the sound is not loud, a backfire is a more severe problem, and you need to address it. Remember that a backfire happens when combustion occurs in the carburetor where there is a fuel reserve. Therefore, you need to fix a backfire as soon as you diagnose it, as it can blow out the combustion system. 

Why Does Your Generator Make a Popping Sound?

If your generator pops during normal operation, it means that there is quite some issue with the combustion system. Often, the pop sound can be due to a poor air-fuel mixture. It could also be that some of the parts of the combustion system are dysfunctional. 

Sometimes, popping is not a huge problem. But you shouldn’t ignore it as it could exacerbate a huge challenge. The best remedy is to invite a mechanic to check out the possible causes of the pop. It should be easy to detect as most pop issues revolve around the combustion of oil. 

Why Does Your Generator Backfire When You Shut It Off?

Some generators will backfire when you shut them off. But this is a minor issue that you can fix without too much hassle. 

Typically, a generator backfires when you’re switching it off, especially if it is running at a high RPM. What happens is that as the generator goes off, there is still a residual motion of the piston. The momentum will cause spontaneous combustion of the air-fuel mixture in the combustion chambers resulting in a pop sound. 

You can solve the issue by bringing the generator to an idle mode for a while before shutting it down. 

Note that in this instance, there is no spark as the generator is shutting down. As a result, the air-fuel mixture that enters the combustion chambers will not ignite. Consequently, it will pass to the exhaust chambers unburnt. There is a supply of fresh air in the exhaust, which may prompt spontaneous combustion, causing backfire. 

The degree of the backfire will depend on the degree of heat in the exhaust chambers. It usually is not too loud as the residual momentum is not as high as the regular running of the piston. You may also witness a few sparks, but they shouldn’t scare you as the issue is ordinarily harmless. 

How to Prevent Backfire When Shutting Off a Generator?

You can prevent such a backfire by not shutting the generator’s engine abruptly. Instead, switch all the appliances running on it and allow it to remain for a while before shutting it down. A minute span is enough to allow sufficient readjustment of the fuel supply to the combustion system. 

Your generator could also be backfiring as you turn it off due to fuel additives and using contaminated fuel. Also, avoid ethanol blends as they will hamper the functioning of the combustion system. Using a fuel stabilizer is still acceptable. Nonetheless, you must ensure that you do not exceed the recommended amount. 

What’s the Best Way to Turn Off a Generator?

You can directly shut your engine without needing to carry out any preventive measures. However, this is only applicable when you’re shutting the generator only for a short while. But if you want to shut it and keep it out of use for some extended period, you must be keen on following some particular precautions. 

Shutting the generator outrightly by pressing the shutdown button then storing it for months will bring problems next time you want to use it. Hence, you need to drain all the fuel from it. Even if you remove all the fuel from the tank, there will still be some left in the carburetor. 

Therefore, you need to shut the fuel valve while the generator is still running to expel everything out. It will use all the fuel that is in the carburetor before shutting down. After it shuts, pull the recoil cord for some time. The aim is to expel all remaining fuel from the system. 

But why is all this necessary? When you leave some fuel in the generator, it will form some sludge. It may also, with time, destroy the float needle valve of the carburetor. Therefore taking it off is necessary before storing the generator for some extended while. 

Final Thoughts

Having your generator in good condition is essential as it will not let you down when you need it most. Hence, among the critical imperatives of ensuring that it’s in good condition is understanding why it is backfiring. 

There are several causes of the issue. However, you should differentiate between backfiring and after firing as they are two unique although related issues. After firing is not too severe. But a backfiring problem needs to be sorted out fast. 

We have laid down the procedures for rectifying both. We have also given you the probable causes of your generator not starting. Of cardinal importance is to ensure that you heed the advice and take the necessary precautions that we’ve discussed. 


Hubert Miles

I've been conducting home inspections since 2002. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and an FHA 203k Consultant. I started PickGenerators.com to help people better understand whole-house and portable generators.

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