A motorcycle backfire is a loud, startling popping sound that comes from your exhaust pipe when unburned fuel ignites in the hot pipe or intake manifold.
Backfires happen on most bikes every once in a while, but if your motorcycle is backfiring on a regular basis, it indicates a problem somewhere in your air, fuel, or exhaust systems.
This article explores the most common issues causing a motorcycle to backfire. Before we delve right into it, you may want to check why motorcycle backfires.
Table of Contents
1. Running an Air:Fuel Mix Too Rich or Too Lean
One of the leading causes of motorcycle backfiring is a faulty air:fuel ratio. This refers to a situation where the engine is getting too much or too little fuel compared to the amount of air it needs for optimal combustion.
If the air:fuel ratio is disturbed, the engine will not burn the fuel efficiently. The unburnt fuel escapes into the exhaust system or the intake manifold, where it can ignite and cause a loud pop or bang.
To fix an improper air:fuel ratio on a motorcycle, start by figuring out the cause. Some common culprits are:
- Clogged or dirty carburetors or fuel injectors
- Faulty or dirty air filter
- Leaky or damaged intake manifold or vacuum hoses
- Faulty or poorly adjusted sensors (oxygen, mass airflow, throttle position, etc.)
- Improper jetting or exhaust tuning
To prevent an unbalanced air:fuel ratio, maintain your motorcycle well and check for any problems in the fuel, air, exhaust, and sensor systems. Adjust the jetting or tuning for your riding conditions.
2. Infrequent Spark: Faulty Spark Plugs, Wires, or Coil
Motorcycle backfiring often happens because of an irregular spark – when the spark plugs are not firing correctly or on time, leaving unburned fuel in the cylinders. The unburned fuel either explodes in the exhaust or intake or backfires.
- Damaged or dirty spark plugs can negatively affect the spark quality and timing.
- Faulty or worn-out spark plugs, spark plug wires, or coils can cause an infrequent spark.
These parts are responsible for the high-voltage current, which ignites the fuel-air mixture.
Inspect the spark plugs by removing them and placing them against the engine block. If they are not sparking well, replace or clean them. Check the wires and coil for any damage.
You can prevent infrequent spark by properly maintaining your motorcycle and changing the spark plugs regularly.
3. Exhaust Upgrade Without Proper Tuning
Motorcycle backfiring can also happen because of an exhaust upgrade without proper tuning. The new exhaust system may change the airflow and pressure in the engine, affecting the air:fuel ratio and the timing.
This can cause unburned fuel to enter the exhaust system and ignite, creating a loud noise.
To fix improper exhaust tuning, you’ll need to tune your motorcycle to match the new exhaust system. This may involve adjusting the jetting, the fuel injection system, or the sensors.
You or your mechanic may need to use a computer device or software to override the ECU settings and optimize the air:fuel delivery and ignition.
By the way, you can prevent this problem using high-quality exhaust systems designed for your specific motorcycle make, year model, and engine, properly re-tuning your motorcycle after installation.
4. Leaking or Clogged Carburetor
A leaking or clogged carburetor causes motorcycle backfiring by disturbing the air-fuel ratio in the engine.
On older bikes, the carb regulates the flow of air and fuel, but if it is dirty or damaged, it can create a lean or rich fuel mix. In both cases, the unburned fuel can ignite and cause a loud pop or bang.
To fix or prevent a leaking or clogged carburetor, clean it regularly with a carburetor cleaning solvent.
- Follow the instructions on using the solution and removing any dirt and debris from the carburetor.
- Inspect your carburetor for damage or leaks, replacing or repairing it if needed.
- Adjust the air-fuel screw on the carburetor to fine-tune the ratio, or take your motorcycle to a mechanic for professional service.
5. Worn Timing Chain
A worn timing chain causes backfiring by impacting the timing of the valves and spark plugs.
The timing chain connects the camshaft and crankshaft and keeps them in sync. The camshaft opens and closes the valves while the crankshaft moves the pistons. The spark plugs fire at the right time to ignite the fuel-air mix in the cylinders.
If the timing chain is loose or stretched, it can skip sprocket teeth, triggering the valves and spark plugs at the wrong times. This can leave uncombusted fuel in the exhaust or intake, where it can explode and backfire.
Follow the tips below to prevent this:
- Inspect and replace the timing chain for wear or damage to fix or prevent this.
- Check for slack or cracks on the chain by removing the valve cover, referencing your service manual.
- Replace your chain if it’s too long.
- Check the tensioner and guides for wear or damage.
- Make sure you also check out our post on 7 steps you can follow to free a rusty motorcycle chain.
6. Problematic Valves; Timing Issues
Problematic valves can cause backfiring by interfering with the timing of the engine. The valves control the flow of air and fuel into and out of the cylinders, opening and closing at the correct times to match the movement of the pistons and the spark plugs.
If your valves are burned, tight, or sticking, they can disrupt the timing and leave unburned fuel in the exhaust system or the intake manifold, causing a backfire.
You can prevent this using the tips below:
- Inspect and adjust your valves regularly, according to your service manual.
- Check the valve springs for any damage or wear.
- Replace damaged valves with new ones.
- Inspect the timing chain and the ignition system for any issues that could affect your engine timing; there’s no shame in asking a mechanic for help.
7. Faulty Fuel Pump
A faulty fuel pump can cause backfiring by affecting the fuel supply to the engine.
Note that the fuel pump delivers fuel to the engine at a certain pressure and volume. If your fuel pump is damaged, it can reduce the fuel flow and pressure, causing the engine to run lean.
A lean fuel mix results in uncombusted fuel escaping into the exhaust system, which can ignite and cause a backfire.
Follow the tips below to prevent this:
- Check the fuel pressure and volume with a gauge, comparing it to the specifications in your service manual.
- If the pressure or volume is too low, replace the fuel pump.
- Inspect the fuel filter for dirt or debris clogs; replace it if needed.
8. Clogged Fuel Filter
A clogged fuel filter also causes backfiring by restricting the fuel supply to the engine.
The fuel filter is responsible for filtering out any impurities from the fuel. Still, if it’s old or dirty, the clogged filter reduces fuel pressure and flow, causing the engine to run lean.
This often results in unburned fuel escaping into the exhaust system, which can ignite and cause a backfire.
To fix or prevent a damaged fuel filter, replace it regularly, according to your service manual. Use high-quality fuel that meets the specifications of your motorcycle.
Always note that a clean fuel filter ensures a proper fuel supply and prevents backfiring problems.
9. Bad Fuel Causes Poor Combustion or Fuel Deposits
Contaminated, stale, or low-quality fuel can cause backfiring by impacting the combustion process and leaving fuel deposits in the engine. This prevents the fuel from igniting correctly, resulting in uncombusted fuel escaping into the exhaust system causing a backfire.
Bad fuel can also leave deposits on the valves, pistons, or spark plugs, which can interfere with the timing and efficiency of the engine.
Follow the tips below to avoid this:
- Always use high-quality fuel that meets the specifications of your motorcycle.
- Avoid storing your motorcycle with fuel in the tank for long periods, as gasoline degrades over time.
- Clean your engine and fuel system regularly to remove any deposits or impurities.
- Drain contaminated or expired fuel and replace it with fresh.
10. Exhaust Pipe Is Too Short
A short exhaust pipe can cause backfiring by creating too much pressure in the exhaust system.
The exhaust pipe is designed to release hot gases from the engine and reduce the noise by providing the gasses with enough length to cool down before they eject.
So, if the exhaust pipe is too short, the gasses don’t have time to cool and rush out while they’re still hot.
A short exhaust pipe can increase the back pressure and reduce the scavenging effect, which is the process of drawing fresh air into the cylinders.
This results in unburned fuel remaining in the exhaust system along with the hot exhaust gas igniting the fuel and causing a backfire.
To fix or prevent a short exhaust pipe from causing backfiring, you should replace it with a longer one that matches the specifications of your motorcycle.
A longer exhaust pipe can reduce the back pressure and improve the scavenging effect and cooling time, which can enhance the performance and efficiency of your engine.
Refrain from using aftermarket exhaust pipes that are incompatible with your motorcycle, as they can affect the air-fuel ratio and cause backfiring problems.
11. Cracked, Punctured, or Loose Exhaust Pipe
A cracked, punctured, or loose exhaust pipe causes backfiring by allowing air to enter the exhaust system.
The exhaust pipe is supposed to be a sealed system that carries the hot gases from the engine through the muffler and out of the rear of the pipes.
However, if the pipe is damaged or not fitted properly, it can create a gap or a leak that lets air in. This often creates a back pressure that impairs the process, causing lean air:mixture and leading to backfires.
If you have a cracked, punctured, or loose exhaust pipe, you should fix it immediately to prevent further damage and backfiring.
- Inspect your exhaust system and look for any signs of damage or wear.
- Check the connections and clamps between the pipe, engine, and muffler and ensure they are tight and secure.
- If you find any cracks or holes in your exhaust pipe, you should repair them with a patch kit or replace the pipe with a new one.
To prevent your pipes from cracking, puncturing, or loosening in the first place, maintain your exhaust system well and avoid any factors that could damage it.
- Inspect your exhaust system regularly and look for any signs of corrosion, rust, or wear.
- Avoid riding on rough roads or hitting any obstacles that could dent or scratch your exhaust pipe.
- Refrain from using aftermarket exhaust pipes that are incompatible with your motorcycle, as they hurt the air-fuel ratio and cause backfiring problems.