Glad That You Asked. Why Do motorcycles Backfire?

A backfire is caused by the buildup of un-burned gasoline in the exhaust header and pipe system. Once the gas has built up to the right mixture with air, it explodes, making a loud bang. A visible flame may momentarily shoot out of the exhaust pipe.

It can happen when you start your motorcycle, for example if you stopped the bike with high rpm’s and extra gas was left in the carburetor, when you accelerate or let off the throttle. Typically backfiring results from various malfunctions related to the air to fuel ratio.

A backfire can occur in carbureted engines that are running lean where the air-fuel mixture has insufficient fuel and whenever the timing is too advanced. As the engine runs leaner or if there is less time for the fuel to burn in the combustion chamber, there is a tendency for incomplete combustion. The condition that causes this is a misfire. The result of a misfire or incomplete combustion is that unburned fuel is delivered to the exhaust where it may ignite unpredictably. Another backfire situation occurs when the engine is running rich, meaning with excess fuel, and there is incomplete combustion, with similar result. Backfire is rare with fuel-injection and computer-controlled fuel mixtures.

7 Responses to “Glad That You Asked. Why Do motorcycles Backfire?”

  1. 1 Dyno Dan Jun 12th, 2010 at 10:41 am

    Actually, deceleration popping (backfire) is caused by a lean condition.
    There is always some unburnt fuel in the exhaust gases. (we never have perfect and complete combustion, if we did the EPA would love it) These leans gases are above the flash point of the gasoline. So if the unburnt fuel gets next to any free oxygen before it cools below the flash point, it self ignites and we have decel popping.
    This is why when you install a free flowing exhaust system on a engine you often aquire decel popping. It lets reversion air up the pipe where it was too restrictive in the stock exhaust system to get in before.
    That is why when you richen the mixture decel popping goes away.

  2. 2 Kirk Perry Jun 12th, 2010 at 11:39 am

    I learned how to control my muffled back-fire flame emitting from the stock-style muffler tip, which was caused by cold air venting through my wear reduced diameter of panhead exhaust spigots.
    If I let the motor wind out just far enough and then was slow to release the clutch, and paused between shifts, I would get an impressive flame. It was a rarely used crowd-pleaser, nothing more.
    But, I had control over it.
    I’ve lost that since I installed re-manufactured heads. I miss it.

  3. 3 Kirk Perry Jun 12th, 2010 at 11:40 am

    Nevermind the cold air swooping in to warp my valves. Cool is cool.

  4. 4 nelu banica Jun 13th, 2010 at 11:14 am

    I learn to provoke these sound on my Sportster, especially when i passed through dangerous car drivers because “loud pipes save lives”. Work each time and is legal.

  5. 5 A 1 cycles Jun 14th, 2010 at 2:13 pm

    HEY “DYNO DAN” then why do we sometimes take all the fuel away , basically shutt off the injectors on some bikes with a -20 percent fuel in the 0% (closed throttle) decell area to get some bikes to stop popping? works for us…almost never have i had to addd fuel to an injected bike to stop decell pop.

  6. 6 Dyno Dan Jun 14th, 2010 at 8:03 pm

    Yes, if you can get complete or near complete fuel shut off then there is nothing to burn so no decel pop.
    The thing about shutting the fuel down is you sometimes feel as if the bike has quit running or shut off on decel at 0% throttle and possbly a little surge when it comes back on. Not a problem other than it can feel weird.
    I have seen time were I can’t remove enough to stop the popping.
    But if it’s working for you, run with it.

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