Why Does My Harley Backfire?

harley exhaust
Robert asked:

My Harley Davidson starts and idles great, but as soon as I give it any throttle to speak of it will spew fire and pop from the exhaust.

It sounds terrible, is very embarassing and I am wondering if it is hurting the engine.

Thanks in advance for any help you can give.

Our Answer

Robert....it's very difficult to answer a question like that without seeing the motorcycle or being able to observe it.  I would have to guess that you are probably experiencing a rich condition to some degree.  Depending on the bike you may need more accelerator pump or you may need some work on the PCM. Too many variables.

I'd like to help but given the information, it's hard to say.  We would need to know whether it's been modified in any way...did you install cams in it?  Any engine work done recently?  How does it run the rest of the time under load?  Etc.




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8 Reviews to “Why Does My Harley Backfire?”
  1. James B Says:

    Burning to rich

  2. Clint M Says:

    Cool.

    It could be an ignition timing issue, running a bit rich, or even a valve timing issue created by a littel bit of exhaust valve overlap.

    Most likely just running a little rich. No real problem with that and can be adjusted to lean it out. Don’t go too far though. Rich is okay, too lean can be catastrophic.

    Besides people pay good money for flame thrower kits. They’re cool at night.

  3. JeffyB Says:

    50 yr old engine technology

  4. REDNECK HOG Says:

    PROBALY RUNNING RICH AS YOU HAVE BEEN TOLD .BUT DONT LISTEN TO JEFFYB HE PROBALY RIDES A JAP CAN.

  5. Bad Mother Clucker Says:

    Good lord, adjust or rejet your carb, your running way too rich.

  6. Skulls Says:

    You did say type or year, efi or carb, soanyway if you get to much excess fuel or oil in your exhaust the heat from the pipe will cause cookoff and inite causing a flame, causes: jetting,blowby,or evensticking valve,normally on a H-D if you have drags and are a little rich it blow it out and not pop or flash till you let off, burning what is left

  7. xjr1300 Says:

    ive got no idea, but when you find out let me know, maybe i can get my yamaha to do it

  8. Rev. Os. Says:

    Ok here is the correct answer, and rest assured that this will not be expensive. I had this exact same problem just weeks ago on my old shovel head.I tried everything, new plugs, wires, carb intake boots, timing, lifter adjustment, and it still did this only when i reved it up. it idled fine.Your jets are fine if it ran good before so don’t listen to all the garb.Here is the answer bro believe it or not I am 90 % sure it is your key switch. It don’t get enough power to the coil because they go bad from vibration. Here is how to check, run a jumper wire from the hot battery post to your coil and see how it runs then. If it runs good then, replace your switch they are 23 dollars. Do also let me know what you find, I’m at