Why are Flames Coming Out of My Motorcycle’s Exhaust?

Do you hear popping or see fire coming from the exhaust on your motorcycle? That’s a backfire caused by unburned fuel finding its way into the exhaust and igniting. 

So why is this happening? Here’s a look at three reasons why your motorcycle may be backfiring.

Motorcycle is Running Rich

Combustion takes place inside a motorcycle’s engine when air and fuel mix and are then ignited. The mix of fuel and air is a delicate balance: too much (or too little) of either and the engine won’t run correctly. 

If the cylinder has more fuel than the available air can mix with, the motor will run rich. That leftover fuel needs somewhere to go, and that’s out of the exhaust valve in the cylinder and down the exhaust system. Once that fuel is exposed to the intense heat of the exhaust system and reaches the outside air, combustion occurs and you get the bang and the flames.

Motorcycle is Running Lean

If the balance is too far in the other direction, you end up with more air than fuel. This can also cause your bike to backfire. When a lean mixture is ignited in the cylinder, it burns slower than a mix with the correct amount of fuel. And when the exhaust valve opens, that unignited fuel and air mix is forced out the exhaust valve and down the exhaust system, resulting once again in a backfire.

Motorcycle Backfires from Too Much or Too Little Fuel

A backfire from a mix that’s too lean or rich can be traced back to a few root causes. A dirty carburetor can cause either a rich or lean condition, so cleaning the carburetor on your motorcycle can help. 

Also, you may want to take a look at the air filter in your motorcycle. A dirty air filter can cut airflow, meaning you end up with more fuel than air in the combustion chamber. Conversely, an air filter that’s damaged can let too much air in, meaning there’s not enough fuel in the combustion chamber.

Short Exhaust Pipes

There’s one other thing to consider: the length of your motorcycle’s exhaust pipes. Some states have laws that require a longer exhaust pipe to prevent backfiring. That’s because shorter pipes can be the culprit of noisy backfires. The air-fuel mixture isn’t the issue here. Instead, shorter pipes sometimes aren’t equipped with a built-in baffle to cut noise from a backfire, not unlike the muffler on a car. Slip-on exhaust silencers can do the trick here, as they’re often equipped with exhaust spark arrestors to decrease the risk of fire.