If you know anything about motorcycles, then you must know that they can backfire sometimes. This is an issue that I’ve had more times than I wanted. Not only is it an annoying issue, but it can dangerous. If a motorcycle backfired in an enclosed space, it can damage your ears. This is why it’s important to figure out why your motorcycle is backfiring if you don’t already have the answer.
The most common reasons for a motorcycle backfiring is due to too much or too little fuel, a loose exhaust pipe, a bad fuel pump, or a dirty carburetor. Additional reasons could include an airbox leak or the use of an aftermarket exhaust.
You may also hear backfiring referred to as afterburn because the fuel combusts outside the engine cylinders after the primary combustion from compression has occurred. If you’re looking for solutions for your backfiring motorcycle, look no further than this article. I’ll go through all the reasons for backfiring and how to fix them.
Why Do Motorcycles Backfire?
Motorcycles backfire when combustion happens outside of the combustion chamber. Your bike is essentially burning fuel and exploding where it shouldn’t be. You may also notice strange flames coming out of your exhaust. This means that the combustion is happening in there rather than in the combustion chamber.
Motorcycles can backfire for various reasons including:
- Too much fuel
- Too little fuel
- Too short of exhaust
- Intermittent spark
- Incorrect timing
- Loose exhaust pipe
What Are the Most Common Causes of Motorcycle Backfire
There are several reasons why your motorcycle could be backfiring. One of the most common is due to too much fuel, meaning the bike is running rich. When the combustion process takes place, it needs to have a certain amount of air and fuel. If there’s too much of either one, your bike will backfire and it can cause damage to the engine. Of course, there are plenty of other reasons why your bike is backfiring.
Inside of your carburetor are jets. When these jets are clogged, it means that not enough fuel is being delivered, so there’s too much air in the mixture. If the jets are clogged with deposits, no fuel can pass through them. The way to fix this is to re-jet the carburetor. It can be a tad complicated if you do it yourself, but it’s necessary to keep your motorcycle performing well.
There are three types of jets in your motorcycle’s carburetor; the main jet, piolet jet, and jet needle. The main jet controls the amount of fuel when you’re at 80% to a wide-open throttle. The pilot jet controls the fuel level when you’re at idle to 20% throttle and the jet needle controls the fuel when you’re between 20% and 80% throttle.
When replacing the jets, you have to first drain your carb and remove it. This process will differ between motorcycles. Once you remove your carb, remove your hoses. Take a picture of where each goes to avoid the guessing game. Next, select the proper size jet. You can check your manual to see what size you need. A smaller jet number has smaller holes and should be used if your engine needs to run leaner.
Once you figure out what size you need, you can now replace them. You can do this by removing your float bowl and gasket. The main jet will be located in the center of the carb and the pilot jet will be located right next to it. You need to unscrew the main jet and pilot jet and replace them with the new ones.
You will now need to access the jet needle by removing the carb’s diaphragm, spring, and top cap. This will allow you to access the jet needle. Push this out of the diaphragm and locate the clip attached to the notches, which will be near the top of the needle. Adjust the jet clop based on what your motorcycle needs. If you’re running rich, move the clip one tung toward the top of the needle. If your motorcycle needs more fuel, move the clip down one rung toward the bottom of the needle. You can now reinstall all of your pieces and test it.
If the jets aren’t the issue, check your timing.
Timing Is Everything
There are two different types of timing in a motorcycle; electronic or points/condenser setup. Most of the motorcycles manufactured from 1970 and on have electronic timing, which makes adjusting it a lot easier.
Electronic timing makes it easier to troubleshoot a backfiring motorcycle because there are minimal parts to check off as the main issue. Electronic timing works by ending a voltage signal to your ignition coil when one of the cylinders is ready to fire. When the coil receives this signal, it dumps the built-up voltage which is carried through your spark plug wire, through the spark plug, and it turns into a big spark at the top of the cylinder.
Generally, electronic modules do a fine job of sending the firing signal, even when they’re older. If you have a timing issue with an electronically controlled bike, then it’s probably a mechanical timing issue.
The issue could be various things such as the timing chain being one tooth forward or backward from where it should be. It could also be that the exhaust valve is sticking open for a little too long, which can be a common occurrence with older bikes.
If you have an older motorcycle with the points and a condenser, then things could get a little more difficult because not only do you have all the mechanical timing issues to worry about, but you’ll also have to figure out if your ignition is timed correctly manually.
If your bike has a points system, the point has to be manually set by following the instructions from your bike’s manual. However, it can also be done by locating certain timing marks inside your crankcase cover and lining those up. Once these marks are lined up, you have to set the points a specific distance away from each other, but this also depends on the model motorcycle you have since they are all different.
If your timing is off and the spark gets to the end of the plug just a little too late, the exhaust valve in the cylinder head will have already started to open and when the spark plug finally sparks, you’ll hear the loud bang of the combustion and may even see some flames coming out of the exhaust pipe.
The next thing to check is whether the carb is dirty or not.
The Carburetor Is Dirty
The carburetor is one of the main headaches that come with owning a motorcycle. As long as your carb is clean with the correct pilot jet and main jet settings, your bike will be fine. However, if you allow the carb to get dirty from within or have the pilot or main jets off their settings, you’re going to find yourself with a jew headaches to deal with.
If your bike has been sitting, there’s a good possibility that your carb will need to be removed and cleaned before your engine can run smoothly. Fortunately, dirty or offset carbs are limited to older motorcycles. With modern bikes, the DFI that works in conjunction with the ECU has transformed how the air-fuel ratio is calculated and delivered, which completely removed carburetor issues.
In older bikes, when the needle inside the carb gets dirty, it will stick to the walls next to it. When it sticks, it allows more fuel to get by and you’ll end up running rich.
Unfortunately, if the smaller issues aren’t the culprit, then it could be a fuel issue.
There are a variety of fuel issues that you could face with any motorcycle. The most common one is that there’s too much fuel, which means your engine is running rich. Fuel and air need to be in a specific ratio inside the combustion chamber for efficient combustion. This is decided by the manufacturer after extensive research and development as well as rigorous testing. Modern bikes have it programmed into the engine control unit and are done in conjunction with valve and spark plug timings along with other factors. If there’s more fuel than air in the combustion chamber, the combustion won’t be efficient enough. The excess fuel, which is super hot, will travel into the exhaust pipe and combust there as soon as it comes in contact with free oxygen. This causes the backfire.
On the other hand, sometimes a motorcycle can have too little fuel, which means it’s running lean. It runs lean when the fuel-to-air ratio is less than the manufacturer’s set ratio. This can happen when you replace the air filter with a more free-flowing one. Since there is more air than is needed, it flows into the cylinder and there will be incomplete combustion. This unburned fuel and air escape into the exhaust where it explodes and burns, resulting in a backfire.
Running an engine lean can be harmful to the engine and requires immediate attention. Believe it or not, it’s better to run an engine rich than lean.
Sometimes these issues can be caused by a bad fuel pump. If you have a bad fuel pump, then you may end up with more fuel than needed or less fuel than needed. If the fuel pump pumps intermittently, this will lead to power loss.
Lastly, if you use low-fuel grade, it can cause you to have a tank of dirty gas, which then causes issues with your fuel injection, causing a backfire. The simple solution to this is to use a higher-grade fuel for your motorcycle. Using high-grade fuel will give you cleaner fuel lines and a cleaner tank of gas.
If this checks out, you may have an airbox leak.
One of the most common reasons an engine may be running lean and backfiring is due to an airbox leak. A leaking airbox increases the air-fuel ratio because there’s too much air and not enough fuel. You should ensure that the rubber packing of the airbox is still in good condition and that the screws that hold the two halves of the box are tight enough.
Exhaust Header Issues
The exhaust header is the metal pipe that is connected directly to the engine. There is also an exhaust pipe that’s the last foot or two of the original pipe that can be disconnected. A loose header is a section directly next to the engine where the hot fumes are sent out of the exhaust valve right after being combusted.
If there’s a hold anywhere between the cylinder head and exhaust header, the fumes, which are at a higher pressure than the air around us, will escape and create a backfire. The best way to alleviate this is to tighten down your exhaust headers. If a bolt is broken, you may require the assistance of a welder.
If you have an after-market exhaust, this could be an issue in itself. Some exhausts are carefully calculated in length. The girth on the inner cavity, angle of elevation, and provisions of the baffle are all designed for optimization. You can’t just replace an exhaust. Changing it will require re-jetting the carb in older models and remapping the ECU in modern ones. If you don’t correct the EC after an exhaust change, it can lead to a rich or lean situation.
Sometimes, the issue is just due to too short of exhaust pipes.
Too Short of Exhaust Pipes
If you have an exhaust pipe installed that’s too short, it can become troublesome. Did you know that most states have laws about how long your pipes must be due to the popping and banging that they can cause? It’s true you know. Short pipes, also known as a shorty, are generally 12 inches or less in length and are intended to give your bike a cleaner look and louder sound. However, a shorty that’s too short will cause backfiring. Shorty exhaust pipes are desirable, giving the bike a simple and small design, but you should never go shorter than 12 inches because of its loud sound effects.
When pipes are short, they usually don’t have a built-in baffle, which is a small section of pipe that acts like a muffler would on a car. It can convert the turbulent flow of the exhaust into a more laminar and quiet flow.
Shorty exhausts tend to backfire more because there’s not as much pipe length for the turbulent air to become more smooth as the bike was originally designed for.
Sometimes, it may be a spark plug issue if nothing’s wrong with your pipes.
Spark Plug or Ignition Coil Intermittent Spark
The spark plugs inside of your bike are responsible for igniting the fuel-air mixture. The timing and sequence of the firing need to be precise to get the best performance out of the motorcycle. A faulty spark plug that isn’t firing or fires off timing does two things. The first is that it leaves unburnt fuel and second, it results in loss of power. If you seem to be having a loss of power as you accelerate, then an intermittently firing spark plug could be the culprit.
An intermittent spark can also be caused by the ignition coil. To check the spark plug, you can unscrew it from the cylinder head and hold the bottom point of the spark plug against the side of the engine. This will ground the plug wire and give the spark somewhere to go. If you don’t do this, then it will try to ground through you and can cause serious shock.
In addition to this, the pick-up coil, pulse generator, or source coil can be an issue. The pick-up coil provides the timing signal for the ignition system. If this isn’t working, the spark plug won’t spark and the bike won’t run at all. You can use a multimeter to check the Ohms to ensure all is well.
Is Backfiring Bad For Motorcycles?
The backfiring itself won’t harm the bike, but what’s causing the backfiring can. Backfiring is a symptom of various issues including poor fuel, overheating, exhaust issues, dirty carburetor, gummed-up intake, bad fuel filter, poor-quality fuel, and so much more. However, the most common cause for a backfiring motorcycle is too much or too little fuel, meaning it’s either running rich or lean. If anything, you want it to run rich as it causes less of an issue in the long run.
It’s important that you ensure all of the specifications from your user manual are being met. They provide you with the exact timing numbers, positions, fuel needed, and more. If you’re changing out your exhaust, it has to be one that works with the specifications of your bike.
Can I Install New Exhaust Pipes?
You can install new exhaust pipes, but you have to correct the fuel-air ratio in the carb or in the ECU. If you don’t, it’ll lead to a rich or lean situation. Fortunately, some after-market exhausts are designed specifically with the model and make of the bike in mind. You should only fit approved after-market exhausts to your bike and remap or re-jet as applicable.
Can Low Oil Cause Backfire?
No, low oil will not cause backfiring. However, low oil can cause a variety of other issues that could potentially lead to backfiring. Riding a motorcycle with low oil can damage the transmission components, clutch plates, and engine. It’s much like a car. If you run it without oil, it’ll probably blow up.
Why Is My Motorcycle Popping on Acceleration?
The popping sound you hear on acceleration is caused by unburnt fuel that’s ignited in the exhaust. This is generally caused by your motorcycle’s engine running too rich or lean. So either your bike is getting too much air or too much fuel. However, there are other reasons why this could happen including a bad fuel pump, airbox leak, bad fuel filter, dirty carb, and loose exhaust pipe.
As you can see, there are a lot of reasons why your motorcycle could be backfiring. Out of all of those reasons, running rich and too short of exhaust is the most common. If anything, you want your motorcycle to run rich because it causes fewer issues than one that’s running lean. There are a lot of problems that come with owning a bike. Fortunately, this is one of the easiest to fix.
I hope that you were able to gain some insight into why your motorcycle may be backfiring. It’s an annoying issue that could even get you into trouble with law enforcement depending on the state you live in. If your motorcycle is backfiring, it’s best to get it fixed as soon as possible to avoid further damage to your bike.
I’m William Guzenski, ASE certified master automobile technician & automotive expert. I love to attend race events and car shows throughout the country. I also loves to travel 40-foot motorhome, exploring abandoned mines and ghost towns. I’m currently building another car for Bonneville Salt Flats and will be campaigning a drag car at several events.