You’ve heard it. At first it was amusing and a sign of something tough and counter culture. Then, after a bit more riding, you ponder why your motorcycle is making all that noise. POP POP POP. Here are the main reasons a motorcycle engine will backfire.
The backfire comes from an explosion or combustion outside of the main combustion point. It is usually due to one of the following :
- Timing
- Lean or Rich Fuel Ratio
- Valve/Head Damage
- Aftermarket modifications to the the air/fuel system (Air intake, carbs, exhaust)
What Is A Motorcycle Backfire?
To understand what a backfire is, you need to understand the function of an engine. Basically, the engine pumps air in, mixes it with fuel at some point, and then releases that explosion and spent gas out the exhaust. This basic process is true for any internal combustion engine. Pistons move inside a cylinder, compressing the air. That air is mixed (at some point) with fuel, and eventually spark is added. This is what moves the piston back down and continues the transfer of energy.
If there is anything abnormal in that process of air pulling into the engine, combusting, and then expelling, you have the potential for explosions outside of the cylinder. These can happen as far up in the sequence as the air box and as far down as the exhaust.
Things like adding or removing your stock airbox can change the amount of air that is sucked into your engine. For an old motorcycle, this could be something done due to broken parts or a change of style (cafe racers for example). Changing the exhaust system or design can also cause that balance of air and fuel to be an issue. One common modification that is done on motorcycles is to put a lighter, race inspired muffler on . This , in my case, was the reason my mildly loud Kawasaki Concours turned into a loud and barking beast. This was after using a Two Brothers exhaust. Very pleasant change in sound, however it required engine adjustment to minimize the popping.
More often than not, you may hear it when a hard revving engine lifts off the throttle. This will cause exhaust backfires if conditions are just right. If you’ve heard street racers late at night , you’ve most likely heard the POP POP POP that happens. In some extreme situations, this may even be visible by looking at the muffler. Flames may be present at the moment of explosion.
The engine also needs to know when to provide spark to the cylinder. If this is off in any way, it could put spark in very inopportune times. This can cause popping, unexpected high revving , or a general inability to idle.
In some cases, the engine’s internal components (valves, pistons, gaskets) do not allow for efficient air flow and combustion.
Will A Backfire Hurt My Engine?
If your engine is running normal otherwise, the occasional backfire will not actually hurt anything. The backfire however is likely a result of poor air to fuel ratios or even timing. This means that the engine is not working the way it was designed from the builder. Lean or rich conditions will have effects on both engine power and engine temperature. Having an engine run hot can be devastating to your motorcycle. It mild cases, it may be just a loss of power. In extreme cases, it can cause complete failure of the engine (cracking of the case). In one adventure I had on my old 1980 GS 550, I experienced the loss of power due to engine overheating. In this case, it was not air/fuel, but a seizing rear brake increasing the workload on the engine. So, in actuality, a backfire can harm your engine in a round about way.
What Can I Do To Fix It?
If you are experiencing the backfire, the first thing to do is retrace anything you might have done . Have you serviced your motorcycle recently Have you removed the air box, carbs, or muffler? In some cases , to quiet the sounds is as simple as adjustment or tightening of the components. Eliminating any gaps, loose connections, or other errors in reassembly.
If you have a carbureted engine and have not serviced it in many thousand miles, it may be a sign that your fuel or air pathways inside the carb are fouled (or dirty). This can be a great project for a new mechanic if you have access to a repair manual and a computer. For an old Suzuki, you may find sites like GSRESOURCES to be a life saver. By cleaning out all the carbon and old fuel deposits, you can bring new life to your engine. (And eliminate the popping)
If you have engine timing issues, a service to adjust points (adjustable) or tuning via computer may be needed. For more modern motorcycles with an on board computer, tuning modules are made to dial in both the timing and the air fuel mix. They simple plug into the computer (ECM) module and problem solved. If you have an older vintage motorcycle, to adjust things may only require a simple turning of a screw (or 4). Your service /repair manual can walk you through this. (Along with sites such as GSRESOURCES).
When in doubt, a visit to your local repair shop or dealer may be needed.