You will hear popping and backfires on some motorcycles as you close the throttle. Some people like this sound and try to encourage it, although people walking along the sidewalk may not be so impressed!
Leaving aside the noise, does a backfire damage your motorcycle, and how do you fix the problem?
The potential damage depends on the level of backfire. Improving the situation usually involves some work on your muffler and the carburetors, if you have them.
What Causes a Motorcycle to Backfire?
The cause of backfires and popping is unburnt fuel escaping from the engine and igniting in your muffler system.
The backfire may also emerge from the carburetor intake on motorcycles with carburetors. In both cases, you might see flames as the fuel is burnt.
But what causes unburnt fuel to escape from your engine?
An imbalance in the fuel-air mix is the source of the problem, and the most common causes are poorly set up carburetors or injectors and air leaks in your muffler system.
Poorly set up fuel supply
The majority of modern motorcycles now use fuel injection instead of carburetors.
Your fuel injection system is far more accurate at sending the correct amount of fuel into your engine than carburetors. However, as bikers, we love to modify our motorcycles, and slip-on end cans are one of the most popular.
End cans vary in quality and how they affect the engine and fuelling, but in most cases, they reduce the muffler’s backpressure, allowing more unburnt air-fuel mix into the muffler.
Electronic fuel injection systems can usually cope with end cans to a certain degree and adjust the fuelling, but significant differences in fuelling will result in popping and backfires.
To cure this problem, you can fit an aftermarket fuel tuner, such as a Power Commander, or take your motorcycle to a dyno and get them to remap your fuel injection system.
If your motorcycle has carburetors, you must check the settings and ensure the carbs are working correctly. This is a whole separate article.
Air leaks in your muffler
The second common cause of popping and backfiring is a leak somewhere in your muffler system. A leak reduces the backpressure in your muffler, like a slip-on end can, allowing more fuel to reach the muffler and ignite.
To find the leak, make sure any gaskets are in good condition, mainly where the muffler headers connect to the engine. Any other joints, such as between end cans and header pipes, should be checked carefully.
This video from Doc Harley explains more.
Are Motorcycle Backfires Bad for Your Engine?
The quick answer is, possibly! Mild popping on the decel when you close the throttle is not likely to damage your engine. However, a lot of popping and banging, with the occasional loud backfire, is of more concern, and you should investigate the problem.
Motorcycle manufacturers design out popping and backfires
Motorcycle manufacturers spend a lot of development money creating and testing muffler systems. This is partly to meet environmental legislation and build the engine performance they are looking for.
Damage – Blueing
Igniting the fuel-air mixture outside your engine means all that heat and energy are expelled somewhere it shouldn’t. Your muffler isn’t designed for those extreme temperatures, and a sign of damage is the muffler system “blueing.”
Except in extreme cases, the backfire is not likely to damage your engine internals, but the performance of your motorcycle will suffer, and fuel consumption will increase.
How Can You Cure Motorcycle Backfires?
The first task is to locate the source of the problem. If you have fitted aftermarket end cans, this is the first place to check.
The end cans themselves may be the issue, or it could be an air leak where they attach to the downpipes.
Check the end cans
If you suspect the end cans, make sure the joint between the can and downpipe is not leaking.
Assuming the connection is fine, put the original end cans on, if you have them, and take the bike for a test ride. If the popping has stopped, you can safely say it’s your end cans.
What you do next depends on your view of the sound. If you don’t like the sound, then either leave the standard end cans on or get your fuel injection system remapped.
Look for air leaks in your exhaust
Having ruled out the end cans, it’s time to check the muffler joints. The most common source of leaks is at the headers, where they join the engine.
There is a lot of thermal expansion and contraction here, and the gaskets inside the header pipes need to be in good condition.
Follow the muffler route, and look at each joint in turn, tightening as necessary. Some contorted muffler routes will have multiple joints to check, particularly on V-twins and V-fours.
What is the difference between Backfires and “Popping”?
There is no difference in the cause of backfires and popping. If your engine is “popping,” there is a minor problem, and if you like the sound, it’s probably safe to ignore it.
If your motorcycle has a severe backfiring problem, we recommend investigating or taking it to a mechanic to check it over.
The video below shows a BMW with an Arrow muffler popping and shooting flames.
Even if the sound is just a mild popping noise, we still recommend that you check your spark plugs regularly.
The color of your spark plugs is a good indication of how your engine is burning fuel. With a good condition engine and fuel mixture, your spark plugs should be a tan or light grey color.
As bikers, most of us appreciate a well-tuned motorcycle and the sound of an engine, particularly with a non-standard muffler. Muffler popping on the decel is music to our ears, but many worry about the damaging effects on your motorcycle.
In most cases, except for a bit of blueing around your exhaust, there is little to worry about, but if you are concerned, get your bike to the tuning shop and put it on the dyno to check.
Also read our article about 9 ways to make a motorcycle exhaust quieter.