Why Does Motorcycle Backfire: (10 Causes & How To Fix It)

Why Does Motorcycle Backfire (Causes & How To Stop It)

Motorcycle backfire also known as afterfire/afterburn can be heard through a loud popping sound (loud bang). When you are decelerating the backfiring sound from the motorcycle can be easily heard, and there are multiple causes for it and a proper way to fix it to avoid possible engine damage. 

A motorcycle backfire is excessively loud one or two popping sounds your motorcycle makes when you decelerate. The most common reason for a backfire is unburned fuel igniting in the exhaust pipe, which is caused by a faulty fuel/ignition system, bad camshaft or ignition timing, uneven air/fuel ratio, or exhaust leak.

This article will be very insightful for people who want to know everything about motorcycle backfires and what happens to the motorcycle if it keeps backfiring.

Other questions such as why it happens, what are the causes/reasons behind it, how it is different from a normal decel pop sound, and how to stop/fix motorcycle backfire will be clearly and concisely answered.

And also with some maintenance and prevention tips your motorcycle’s engine will be safe from any possible damage from backfiring.

What Is Motorcycle Backfire?

Motorcycle backfire is the explosion sound made during the combustion process in the exhaust system instead of the combustion chamber.

During a compression process, the intake, compression, ignition, and combustion/explosion of the air/fuel mixture are all done inside the combustion chamber to generate power for the motorcycle to run.

So when a motorcycle backfire makes a loud explosive popping (gunshot-like) sound, it simply means some of the unburned fuel get’s passed through the exhaust valve into the exhaust pipe where it will come in contact with outside air which leads to a loud explosive motorcycle backfire sound.

These loud motorcycle backfires can lead to potential damage to the engine, exhaust system, or valves of the motorcycle.

So now you know what motorcycle backfire is and it’s a technicality, we will now move on to why might be your motorcycle backfiring.

Reasons Why Your Motorcycle is Backfiring?

The reasons why your motorcycle is backfiring are always related to faulty fuel system components, ignition system components, air supply unit, or mistimed camshaft or valve opening.

For a motorcycle to run properly and smoothly it needs a steady supply of fuel and air and a proper combustion cycle/process to generate power.

Faulty/bad components of the ignition system or fuel system will cause imbalance and inefficiency in the supply and ignition of the air/fuel mixture.

Consequences such as uneven air/fuel ratio will result in an extremely lean or rich mixture or intermittent/weak spark from the spark plug will cause motorcycle backfire.

Now we will look into the causes of motorcycle backfire and how these faulty components of fuel, ignition, air supply unit, or valve movement (camshaft) play a part in it.

What Are The Most Common Causes Of Motorcycle Backfire?

Here are the most common causes of motorcycle backfire in carburetor and fuel-injected motorcycles.

1. Rich-Running Engine (Too Much Fuel)

For every motorcycle to run efficiently it needs a steady supply of balanced air/fuel mixture supplied to the combustion chamber to have a full ideal combustion.

Faulty fuel, ignition, and air supply system components such as bad fuel pressure regulator with high pressure, fouled spark plugs, or clogged air filter will cause the motorcycle to run rich.

Where also the fuel supply will outweigh the amount of air resulting in an imbalance in the air/fuel ratio, hence the motorcycle will run an extremely rich mixture.

2. Lean-Running Engine (Too Less Fuel)

On the opposite of the rich mixture, there is a lean running engine, which simply means there is less amount of fuel compared to air supplied to the combustion chamber.

This will also result in an imbalance in the air/fuel ratio. Low-pressure fuel regulator, clogged carburetor jets or fuel injector, weak fuel pump, or vacuum leak will cause the motorcycle to run a lean mixture.

Faulty/malfunctioning components of fuel, ignition, and air supply unit will work in conjunction to cause a lean running engine.

3. Loose Exhaust Pipes & Headers (Exhaust Leak)

After successful combustion, all the gases are expelled through the exhaust system.

Exhaust leaks will be caused due to loose exhaust headers or exhaust pipe (muffler, middle pipe/cross pipe) which will introduce more outside air into the exhaust system or can go back into the combustion chamber.

And for fuel-injected motorcycles, the extra air from the exhaust leak will trigger the O2 sensors, which will cause an imbalance in the air/fuel ratio.

A blown head gasket will also cause an exhaust leak, so keep an eye out for that too 

This exhaust leaks results in an uneven air/fuel ratio which will cause the motorcycle to backfire.

4. Intermittent Spark (Weak Spark)

An intermittent spark is a condition where the spark plugs won’t fire on time, create a weak spark fire, or don’t even fire at all. This will cause inefficient burning of the air/fuel mixture and a lot of unburned fuel will be expelled through the exhaust.

This will cause a drop in performance, sputtering, or misfiring motorcycle and can also cause backfire as the unburned fuel will get ignited in the exhaust system due to high temperature and outside air getting into the exhaust system.

A worn ignition coil, spark plug wire insulation or the spark plug being damaged itself will cause intermittent/weak spark condition in the motorcycle.

5. Uneven Air/Fuel Ratio

As discussed above, other causes such as damage/leakage or clogging of fuel system components, ignition system components, and air supply unit components will cause the motorcycle to have an uneven air/fuel ratio.

Motorcycles will start to backfire whenever there is an uneven balance of air/fuel ratio in the motorcycle.

6. Out Of Tune Carburetor Or Clogged Fuel Injector

To supply air and fuel to the combustion chamber, a carburetor plays a crucial role.

And a properly tuned carburetor will supply a balanced ratio of air/fuel mixture to carry an effective combustion cycle.

The bad setting of the mixture screw, clogged pilot, main, or needle jet will disrupt the flow of air/fuel mixture.

So clean the jets and properly tune the carburetor for a motorcycle to run properly without causing any performance issues.

7. Clogged Air Filter Or Airbox/Intake Manifold Leak

For a motorcycle to have successful combustion and ignition it needs a sufficient amount of air along with fuel.

A clogged air filter due to dirt and debris buildup over time of a crack to the intake manifold gasket leak will disrupt the air supply sent to the combustion chamber.

This will cause the motorcycle to backfire due to inefficiency and imbalance in the air/fuel ratio caused by a clogged air filter or intake manifold gasket leak.

So clean/replace your air filter from time to time and keep an eye out for any gasket leaks near the intake manifold of the engine.

8. Electronic Or Mechanical Timing Issues

Almost all motorcycles from 1960 to 1970 switched from points ignition system (mechanical) to electronic ignition system, which is controlled by a CDI box and ignition system components such as ignition coil, spark plug wire, spark plugs, pickup coils, or crank position sensors, and other spark carrying components.

A CDI box typically consists of a small transformer, a triggering circuit, a charging circuit, and a main capacitor.

Malfunctioning or faulty parts of the CDI box or ignition system components will cause a timing issue in the motorcycle where the spark plugs will not fire on a given recommended time.

So appoint a qualified mechanic to look at the components of the CDI box and ignition system for any malfunction and replace them if necessary to prevent and avoid motorcycle backfire.

9. Bad Fuel Pump

A fuel pump motor’s job inside the motorcycle is to transfer fuel from the fuel tank to the engine.

The fuel pump motor keeps providing a steady supply of fuel to carry out the combustion process.

The bad or failing fuel pump will fail to transfer the fuel from the tank to the engine.

Failing or bad/clogged components of a bad fuel pump such as pressure regulator, fuel level sensor, and strainers will restrict and fail to supply the fuel to the engine.

10. Low-Grade Fuel (Low Octane)

Lower octane fuel will burn spontaneously even before its fully compressed by the pistons, which builds up high pressure inside the engine and creates engine knocking noise, and will also cause engine backfire.

Lower octane fuel has a low tolerance to a high amount of compression, which will cause the air/fuel mixture to ignition even before the piston reaches its TDC (Top Dead Center) position.

So opt for a higher or recommended octane level fuel for your motorcycle.

Most Common Symptoms/Signs Of Motorcycle Backfire?

Here are the top most common signs of motorcycle backfire when you are letting off the throttle while decelerating or even accelerating and other handy signs you should look out for.

1. Motorcycle Backfire On Deceleration?

Motorcycles backfire during deceleration due to uneven air\fuel ratio where the engine is running a significantly lean mixture, which results in fuel remaining unburned and getting ignited/exploding into the exhaust system once it comes into contact with air.

Loss of compression caused by a header vacuum leak due to a blown head gasket or exhaust leak will cause the backfiring sound in the motorcycle.

2. Motorcycle Backfire On Acceleration?

Though it is rare, motorcycles backfire during acceleration is due to intake manifold/gasket air leak where the extra air is introduced which imbalance the air/fuel ratio. 

Faulty fuel and ignition system components such as carburetor, fuel injector, fuel lines, spark plugs, ignition coil, or spark plug wire will disrupt the amount of fuel required for combustion, and missed spark plug firing rate will cause the motorcycle to backfire on acceleration.

3. Strong Smell & Smoke From The Exhaust?

During a backfire, a lot of unburned fuel gets ignited into the exhaust system rather than into the combustion chamber, which creates a strong smell or smoke of the fuel coupled with the backfire sound.

This sign is pretty easy to spot as you will be able to hear a loud backfiring sound from the motorcycle and some can smell the unburned fuel from the exhaust system.

Is Backfiring Bad for My Motorcycle & Can It Damage The Engine?

If your motorcycle is making a loud explosive backfiring sound, then it will damage the exhaust system and engine of the motorcycle over time. A lot of unburned fuel getting ignited into the exhaust system will create gases which can travel back inside the engine which will cause problems to the combustion process of the engine.

As the burned gases should be expelled from the motorcycle rather than staying inside the engine or the exhaust system.

Carbon buildup and high-pressure buildup from motorcycle backfire inside the engine and exhaust system will cause damage to the parts of these systems.

Is Motorcycle Exhaust Popping (Decel Pop) Bad?

Decel pop makes a series of soft/short popping sounds which is not bad for the motorcycle and is often noticeable with an aftermarket exhaust. But an extremely loud (gunshot-like) explosive backfire sound is harmful to the motorcycle and should be looked into to fix it as soon as possible to avoid engine damage.

So, properly tune the air/fuel ratio of the motorcycle once you install an aftermarket exhaust as it will have much less restriction to the airflow inside the exhaust pipe.

How To Fix & Prevent Motorcycle Backfire?

To fix a motorcycle backfire, you will have to replace/clean the faulty or clogged components of the fuel system, ignition system, & air supply unit, which will help the motorcycle to operate a clean and ideal combustion process with a proper even air/fuel ratio.

To Prevent Motorcycle Backfire, Here Are Some Quick Maintenance Tips.

  1. Clean your air and fuel filter regularly.
  2. Routinely clean your carburetor jets to avoid debris/dirt buildup.
  3. Clean your fuel injector’s nozzles.
  4. Remove the carbon buildup on spark plugs or replace them if damaged.
  5. Always use a high-octane or recommended octane-level fuel.
  6. Make sure there is no debris in your fuel tank.
  7. Inspect for any leak or damage to the intake manifold gasket.
  8. Inspect for any leaks or damage to your exhaust system or head gasket. 
  9. Properly tune the air/fuel ratio of the motorcycle by a professional or by yourself.
  10. Make sure there are no loose fitting bolts in the exhaust system.

Motorcycle Backfire FAQ

What Is The Difference Between Misfire And Backfire?

A misfire happens when the spark plug fails to ignite the fuel/air mixture inside the cylinder, whereas the backfire ignites the unburned fuel from the combustion chamber into the exhaust system while creating a loud backfiring/popping sound.

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