It’s almost certain you have heard a pop or bang (usually loud) from a motorcycle while its engine was running, and you were caught off guard by it.
While it can be loud and scary (if it catches you off guard) — you might wonder what causes it and what it means for the motorcycle.
I have personal experience with pops and bangs, given that my Kawasaki ZX-10R often does so when decelerating. In 19 years of riding, I have heard thousands of pops and bangs and seen exhaust pipes catch fire.
Motorcycle backfire is the loud pop or bangs noise which comes from the exhaust pipe. Motorcycles backfire when fuel combustion takes place in the exhaust pipe.
The main reasons for motorcycle backfiring are too much fuel, too little fuel, loose exhaust pipe, use of aftermarket exhaust, a bad fuel pump, Airbox leak, bad fuel filter, and a dirty carburetor.
Motorcycle backfire is also referred to as afterburn because fuel combusts outside of the engine cylinders and after the primary combustion from compression has already occurred. All fuel combustion should only occur within the engine cylinders during the compression cycles. Any fuel combustion outside of the engine cylinders
Too much fuel (engine running rich)
For a motorcycle engine, fuel and air need to be in a specific ratio inside the combustion chamber for efficient combustion. The motorcycle manufacturer decides on this specific fuel to air ratio after extensive research and development, followed by rigorous testing. With modern bikes, this is programmed into the ECU (Engine Control Unit) and is done in conjunction with various factors such as a valve and spark plug timings, for instance. The fuel and air mixture ratio was set in the Carburetor in older motorcycles. The Carburetor is a purely mechanical device.
If there is more fuel than air in the combustion chamber, the resulting combustion won’t be efficient enough. The super-hot excess fuel would escape into the exhaust pipe and combust there as soon as it comes into contact with freely available and highly combustible oxygen. This would be heard as a loud bang and called a backfire. It is not uncommon to see flames from the exhaust pipe at the time of backfire.
A motorcycle is said to run rich when the fuel to air ratio exceeds the manufacturer’s prescribed ratio. In simpler terms, when a motorcycle engine has more fuel than is needed in the cylinders, the motorcycle is said to be running rich.
It is important to note that running a motorcycle rich will reduce gas mileage.
Tip 👉: Check the spark plugs on an older bike with a carburettor. If the spark plugs seem darkened, you are running the bike rich. The dark soot matter is a telltale sign of incomplete combustion of a fuel-rich mixture.
Too little fuel (engine running lean)
A motorcycle is said to be running lean when the fuel to air ratio is less than the manufacturer’s prescribed ratio. In simpler terms, when a motorcycle engine has less fuel than is needed in the cylinders, the motorcycle is said to be running lean.
This typically happens when you replace the air filter with a more free-flowing one. As more air than is needed flows into the cylinder, there is incomplete combustion. The unburned fuel and air escape into the exhaust, where it explodes and burns. If you change the air filter on your motorcycle, be sure to find out whether the bike needs to be rejetted to increase fuel into the cylinders or if the ECU needs to be adjusted to allow more fuel in.
Running an engine lean can be harmful for the engine requires immediate attention. If anything, it is better to let the bike run rich than lean — if one must choos a lesser devil, i.e.
Tip 👉: Check the spark plugs on an older bike with a carburettor. If the spark plugs seem clean and whitish, you are running the bike lean.
Loose exhaust header pipe
The exhaust header is the first piece of the exhaust pipe. It is that part of the exhaust system that connects directly to the engine casing. If, for some reason, air leaks from the engine manifold where it connects with the exhaust header, you will hear a constant loud and disturbing note. While this will be annoying and ruin the beautiful exhaust note, it should not be considered a backfire.
Most people mistake this exhaust leak for a backfire.
Let me explain why a loose exhaust header pipe will eventually cause a backfire. The exhaust airflow in the entire exhaust pipe is a series of high and low-pressure air flows. During the low-pressure flow, external air will get sucked into the header from where the exhaust leaks. In a motorcycle with a fuel injection system, this sucking in of air will trigger the exhaust oxygen sensors to report more oxygen than is expected. As a result, the ECU will attempt to correct the pseudo “lean” situation by increasing the fuel injection. That, in turn, will lead to backfiring simply because the motorcycle engine will be running rich now!
After market exhaust
The stock exhaust on bikes has a carefully calculated length. The girth on the inner cavity, the angle of elevation of the exhaust, the provisions of a baffle and so on are all designed for optimisation.
So one can’t simply replace the exhaust. Changing the exhaust in most cases will require re-jetting the carb in older motorcycles and remapping the ECU in modern ones.
Not correcting the fuel-air ratio in the carb or in the ECU after an exhaust pipe change will lead to a rich or lean situation — rich if shorter exhaust pipes are being used and lean if the pipe is even longer than the stock pipe.
However, some after market exhausts are designed specifically with the model and make of bikes in mind. On my Kawasaki ZX10R, I run an Akrapovic exhaust system with Titanium headers. Akrapovic clearly states that the ECU will need remapping.
Tip 👉: Fit only approved after market exhausts for your bike. And remap or rejet as applicable as per the exhaust manufacturer’s instructions.
The spark plugs inside your engine are responsible for igniting the fuel-air mixture. Their firing timing and sequence need to be precise to get the most out of the machine.
A faulty spark plug that isn’t firing or fires off timing effectively does two things. First, it leaves unburnt fuel and second, it results in loss of power. So, if you happen to feel a specific loss of power as you roll onto the throttle (basically accelerate), an intermittently firing spark plug could be to blame. The unburnt fuel will, of course, leave the engine when the exhaust valve opens and combust in the exhaust pipe resulting in motorcycle backfiring.
Tip 👉: When was the last time you got your spark plugs checked? Do you know when they are due for a change?
Bad Fuel Pump
If you have a bad fuel pump, then one of two things will happen.
The pump may supply more fuel than needed, which will ultimately lead to the engine running rich.
Some of the excess fuel will escape into the motorcycle exhaust, and as the combustion process takes place, we will hear a loud pop and bang — a backfire.
The other scenario that can play out is the less fuel than needed situation in the cylinder. In the absence of enough fuel, a lean condition will come about. This, again, will lead to a backfire ultimately.
It is important to note that a fuel pump that pumps intermittently will also lead to power loss. This will be picked up by the rider as he rolls on the throttle but feels as if the bike isn’t pulling as much as it should.
It is essential that the fuel pump is serviced regularly, at which times the fuel line is also checked. It is possible that over and above a faulty fuel pump, the fuel injectors might also need a thorough check because an incorrectly working fuel injector will alter the fuel-air ratio in the motorcycle’s engine and lead to a backfire.
Tip 👉: Never let a bad fuel pump issue drag. Always address it immediately because replacing it can be expensive.
One of the common reasons why the engine might be running lean and therefore backfiring is a leaking airbox. A leaking Airbox will increase the air-fuel ratio because there will be too much air and not enough fuel.
To fix this, ensure that the rubber packing of the airbox is in good condition and that the screws clamping the two halves of the box are tight enough.
Also, visually inspect the airbox to ensure that it shows no signs of cracks.
Tip 👉: At least once a year during service as the mechanic opens up the bike and inspects the airbox.
Bad Fuel Filter
The fuel filter makes sure that no impurities present in the fuel pass through into the cylinders. If not replaced on time, it can become too clogged and disrupt the fuel flow. I replace the fuel filter on all 3 of my bikes twice a year. With high-performance bikes, there is usually no option of cleaning the filters. You simply have to replace it.
However a clogged fuel filter, will ultimately create a lean air-fuel mixture situation, and your motorcycle will exhibit various signs to indicate the reduced presence flow of fuel into the cylinders. While one of them is exhaust backfires, you will also notice that idling will not be so smooth. The vehicle may even feel like it wants to stall and begin to cut off on its own.
Tip 👉: Know when the fuel filter is due for replacement. Always replace the fuel filter on time.
The famous carburetor, ladies and gentlemen! The carb singularly has been the boon and the bane of older motorcycles! Run a clean carb with the correct pilot jet and main jet settings, and life is golden. Allow the carb to get even slightly dirty from within or have the pilot or main jets off their settings, and life becomes very hard immediately.
If your bike has been sitting for a while, chances are that the carb will need to be removed and cleaned before you can run a smooth engine.
However, dirty or offset carbs are only limited to older bikes. With modern motorcycles, DFI (Direct Fuel Injection) working in conjunction with ECU (Engine Control Unit) has completely transformed how the air-fuel ratio is calculated and delivered. This has completely removed carburettor problems!
Tip 👉: If you own an older bike, always keep the carburetor clean and jetted as necessary. The carb has very little tolerance concerning impurities and therefore requires regular servicing.
Motorcycle backfire may not necessarily be bad for the bike but almost certainly is for your ears and the general peace and quiet of the surroundings.
Out of the two situations that must be created for the engine to backfire, the lean situation is more dangerous for the motorcycle overall.
Be alert and catch a bad situation before it spirals out of control!