What is causing my motorcycle to stop working? Here are 6 reasons why your motorcycle is backfiring and how to fix it

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Backfire refers to a loud, ringing sound coming from your exhaust pipes while your motorcycle is being driven.  An exhaust backfire from your motorcycle can be annoying and dangerous. It can damage your ears if you’re in a confined space, or could start a fire as some backfires even emit flames, I’ve almost had my hand burned once from a pesky backfire problems I was troubleshooting.

Why does my motorcycle keep crashing? Uncombusted fuel can cause a motorcycle to backfire.  This problem is caused by:

  • Incorrect timing
  • Too much fuel
  • Exhaust pipe loose
  • Too little exhaust
  • Too little fuel
  • Intermittent spark

Over the years I have worked with dozens of motorcycles. Backfire issues are a common problem.  I am a minor automotive engineer and have spent some time in my classes fixing backfires.

I’ll describe each possible scenario as simply as possible so you can troubleshoot your motorcycle and try to understand why you might be experiencing this problem.

Incorrect Timing

When it comes to troubleshooting motorcycle issues, it’s always nice to have the right, basic tools handy so you can make these fixes in the comfort of your own garage. Click here to view my recommendations for tools to fix your motorcycle.

There are two types of timing available for motorcycles: electronic timing and points/condenser.  Electronic timing makes it much easier to operate motorcycles built after 1970.

Electronic timing has made it simpler to identify the cause of a backfiring problem.  Electronic timing works by sending a voltage signal through your ignition coil when one cylinder is in its compression stroke.

This voltage signal is received by the ignition coil. The built-up voltage is then emitted through the spark plug wire.

Electronic ignition modules generally do a great job at sending the firing signal, even when they’re 40 years old.  So if you have a timing issue with an electronically controlled motorcycle then it’s most likely a mechanical timing issue

It could be that the timing chain is one or two teeth forward or backwards from where it should be or that an exhaust valve is sticking open a little too long. This is quite common with older motorcycles.

It’s a classic motorcycle with points and condensers that can make things more difficult. You have to not only worry about mechanical timing issues, but also how your ignition is timed. 

My 1969 Triumph 250 was originally equipped with a points system. I got fed up of it and installed an electronic system. Two small, metal discs are what a point is. They open and close depending on how your engine turns. Each time the discs come apart, it sends a signal to the ignition coil for the spark plug to be lit.

The written instructions provided in your motorcycle repair manual or shop manual will help you to set the point manually.  But usually it’s done by finding certain timing marks inside your crankcase cover and lining those up. Once you have aligned the timing marks, you need to position the points at a specified distance from each other.  This will depend on the model of motorcycle that you have, as each one is different.

If the timing is wrong and the spark doesn’t reach the end at the spark plug enough, then your exhaust valve in the cylinder head will have opened and the spark plug will spark. You’ll then hear the really loud bang of the combustion and you’ll probably see flames coming out of the exhaust pipe unless you have a high quality spark arrestor in your pipes.

Too much fuel

To ensure that the combustion process inside the cylinder takes place smoothly, there must be enough fuel and air.  If there’s too much of either one then the combustion will be weak and has the potential to damage your engine.

You hear it when someone says the term. “my motorcycle is running rich” They believe that the cylinder has too much fuel or not enough air.  An engine in a poor condition is not healthy and should be repaired. Running rich can not only reduce your mileage per gallon but also cause serious backfires in your exhaust pipe.

If too much fuel is in the cylinder at the spark plug ignition, then the spark plug will not ignite all of it. Instead, the combustion process will fail because there will be fuel left behind that is expelled from the exhaust valve. 

The unburned fuel will contact atmospheric air, and then be enclosed by the very hot exhaust header. The fuel will combust when it comes in contact with fresh air and high temperatures. This creates a loud bang or pop sound.

This can be dangerous, I’ve experienced it firsthand.  I once had to troubleshoot a 2-cylinder motorcycle. Each cylinder had its own exhaust pipe.  To check how much air was coming out of each exhaust pipe, I would place my hand at the end and turn.  If one side is stronger than the other, you will know that there is a problem with that cylinder.

As I was reaching down for the air, there was an audible bang and a fire coming from the tailpipe. My hand was only a little over an inch away.  I don’t want to place my fingers in the path of an exhaust pipe on a motorcycle.

Carburetor almost always is at the root of too much fuel in your engine. Unclean carburetors can cause many problems.

The needle in the carburetor tends to stick to the walls if it gets too dirty.  If it sticks, it allows in more fuel and results in a rich condition. If you have sat on your motorcycle for a while, make sure to remove the carburetor from the bike and give it a thorough clean so you don’t have to deal with all the problems that accompany a dirty carb.

Clean carbs can solve the problem of your motorcycle being rich. Check out my article for other solutions to a motorcycle running rich.

Loose Exhaust Header

For clarity, an exhaust header on a motorcycle is the pipe connecting directly to the engine. An exhaust pipe is the pipe that can be unbolted or disconnected.  So when I say a loose header can cause the problem, I’m referring to the section directly next to the engine where the hot fumes are pushed out of the exhaust valve immediately after being combusted.

When I was fixing up a BMW R80 for my friend, the threads at the back of the cylinder head had been damaged.  It was difficult to get the exhaust nut on the cylinder head to fit properly. Finally, we decided to leave it as it was.  When we started the motorcycle, there was an immediate sound coming from the exhaust nut. The exhaust was coming out of the loose exhaust nut.

The engine exhaust gases have a much higher pressure than the surrounding air. If there is even a tiny gap between the exhaust header and the cylinder head, then fumes can escape making a lot of noise.

It is best to reduce the exhaust headers.  Some use nuts and others bolts. Others use compression fittings. If a bolt has broken off then get help from a welder or machine shop to fix the broken bolt for you, but I would not recommend riding much with that problem because it’s only going to get worse.

Some motorcycles vibrate so much while they’re running that over time the exhaust nuts loosen.  You should check the exhaust header connection at least once a month as part of your preventative maintenance.

Too Short Of Exhaust Pipes

Due to the potential for excessive popping and banging, very short exhaust pipes can be problematic.  A short pipe, also called a shorty or a shorty, is usually 12 inches in length. These are designed to give motorcycles a cleaner look with a more louder exhaust. This is just too short and will often lead to backfiring.

The shorty exhaust pipes look great on motorcycles. but it’s a good rule of thumb to never go shorter than 12 inches because of the adverse effects. 

When pipes get really short they usually don’t have a built in baffle. A baffle, a small part of the pipe that functions in the same way as a car’s muffler, is a small piece of the pipe.  It converts the turbulent exhaust flow into a smoother, more quiet flow.

I would always recommend getting an exhaust pipe with a removable baffle, that way if your state allows louder exhaust then you can take it out, but if you’re going on a road trip then you just slide the baffle back in and you won’t have to worry about getting pulled over.

Shorty exhaust also has a tendency to backfire more because there’s not as much pipe length for the turbulent air to become more smooth like the motorcycle was originally designed for. Because motorcycles have a certain length of exhaust pipes, engineers use it to maximize fuel efficiency.

People put smaller pipes on to get more performance. But, with higher performance comes greater risk of backfire.

Too Little Fuel

Although the situation of a backfire due to insufficient fuel may not seem as common, I have seen it before.  This can sometimes happen when people take out their stock intake airbox and replace it with cheaper pod filters (which is what I did). Refer to my guide on pod filters vs. air boxes.

This is when your cylinder has too much air but not enough fuel. Lean conditions in an engine can be very damaging to the internals of the engine.

Your backfiring problem could be caused by too little fuel in your cylinder or an excessive amount of air. The huge volume of air can prevent the spark plug from trying to ignite the mixture of fuel and air. 

Air doesn’t burn, the fuel does. So when there’s way too much air then sometimes the fuel won’t combust. Once the exhaust valve is opened and the mixture of fuel and air hits the hot exhaust header, it ignites and produces a loud bang sound.

You can fix this by cleaning your carburetor jets thoroughly. A carburetor that has even a tiny bit of dirt can render it useless.

To remove the gasoline gelled to the carburetor, you should take it off the engine and carefully wash it with soapy water. If you have used ethanol additive gasoline then you’ll be sure to experience carburetor issues sooner or later.  Ethanol is terrible for carburetors.

Clean all internals of the carburetor. Make sure that you can see all the tiny holes within the jets.  If you can’t see through them then fuel isn’t going to be able to flow through it. You should spend a lot of effort cleaning them. Inline fuel filters should be installed before you start the carburetorsSo they can stay clean.

I have created a video series on motorcycle restoration that includes a detailed 25-minute video on how to clean and rebuild carburetors. The series also covers other difficult-to-tackle parts such as electrical and body work. I give dozens of tips and tricks that you won’t find anywhere else online. Click here if you’re interested in viewing multiple videos that will help fix up your bike or if you’re interested in completely building your dream motorcycle!

Intermittent Spark

A problem that causes intermittent sparks is very irritating.  It’s hard to diagnose and pinpoint, but here are some signs that this might be your problem. 

If your motorcycle only backfires at very specific times, such as when it’s really cold, really hot, when it rains, when you turn left, etc.  The intermittent spark could be an issue.

An intermittent spark is when the spark plug usually fires just fine most of the time, but every once in a while it just won’t spark and your engine will backfire.  When there is fuel and air in your cylinder and the spark plug doesn’t ignite it, it gets pushed out of the cylinder on the compression stroke and as soon as it hits the hot exhaust header it combusts.

Spark plugs, spark wires, and the ignition coil can cause intermittent sparks.  You can check the spark plug by unscrewing the spark plug from your cylinder head. Hold the bottom end of the spark plug against the engine.  This is to ground your spark plug wire and provide a place for the spark to go.

If you don’t hold it against the engine block then it will try to ground through you and can cause a serious burn or shock.  Don’t try this unless you have someone with you who knows how to do it.

Once you’ve got the spark plug in place against the block, hit the start button.  If spark is appearing at regular intervals, the spark plug may be fine.

You can check the spark plug wires by purchasing a spark tester from an auto parts shop for around $10 and following the instructions.  It is also a good idea to inspect the wire visually. You should inspect the wire for cracks and frays. Replace any cracks or frays in the wire.

If you suspect you have a bad ignition coil, it’s best to just simply replace it.

Similar Questions

How do I install exhaust pipes?  Most new exhaust pipes are simple to install.  Simply remove your old exhaust pipes from the header and slide them out. Install the new ones and bolt them on.  Be sure to buy the correct size.

Is backfiring likely to reduce the motorcycle’s fuel efficiency? Although a backfire is not directly responsible for worse gas mileage it can indicate that your fuel has not been converted into mechanical energy properly.  So you will get worse gas mileage when you have a backfire, but it’s not caused by the backfire itself. The backfire is a sign of the problem.

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