Thursday, May 12, 2011

Tech Tip - Backfiring or Popping on Deceleration

More than likely your engine is "popping" not really backfiring. Backfiring, is extremely rare these days with electronic ignitions and the high reliability of engines. Real backfiring means the engine is not properly timed and the fuel/air mix is getting a spark and ignition at the wrong time (when the intake valve is open). Real backfiring is actually the bike "spitting up" in a sense, backwards through the carburetor or intake.


So, the popping you are hearing is not back-firing. It is a popping resonating sound generated by a lean condition on deceleration. When you suddenly close the throttle, the incoming air is choked off and the closed throttle also causes the bike to return to idle on the very lean idle circuit. However, momentum of the bike is actually "driving" the engine in a sense, and that engine is still spinning at rpm that needs far more gas than the closed throttle position can provide. The engine is still spinning but the incoming fuel rate is very low, until just enough builds up in the combustion chamber to "fire" on the exhaust stroke of the engine.

Don't "change the plugs" like some people suggest, it’s a waste of time and money. Messing with the ignition timing really won't help either, because it is still a lean fuel/air mix on deceleration that is the cause. ( I said cause and not problem, because the popping is not at all damaging to the bike).


 Often people think their bike is not tuned properly after the addition of “aftermarket” mufflers because of the additional popping they hear. Stock mufflers provide a lot of volume for the rumbles and popping in the exhaust to be toned down by resonating in the larger muffler canister. Motorcycle builders also have to meet sound regulations along with emission standards. Some of that sound proofing is in the mufflers. But, aftermarket mufflers that offer easier breathing, and are usually lighter and smaller, don't offer that same volume (Less volume = less sound attenuation = more noise and more popping). Modifying the exhaust (especially if it gets louder) will only make the popping louder. Because most aftermarket pipes produce "lower back pressure", to add to the popping; aftermarket pipes also tend to lower back pressure this will cause the fuel/air mix to lean out. So don’t worry about that popping you sometimes hear and enjoy this year’s riding season.

Tony D.
Service Manager
Wild Fire Harley-Davidson


  1. Excellent dear....
    Wow very inspirational. Thank you so much for sharing!


  2. Using after market mufflair. Will it spoil any of the engine part compared to stock mufflair?

  3. Does Harley Davidson encourage using after market mufflair?

  4. Then why does this popping stop after we install a proper after market tuner?

  5. Hey Tony,
    Thanks for that info on the popping. Is there a way to correct the Lean fuel/Air mix on deceleration?

  6. You can reduce/ remove the popping by adding fuel percentage to the tune. If you have a Harley Race tuner then any dealer can do this be adding 5%-10% extra fuel. This will decrease your fuel range and also cause your spark plugs to need to be replaced more frequently. A Dyno tune will also smooth this out but is not always going to remove it completely. The popping will not cause any damage to the engine at all though its purely an issue if the sound annoys you at all.

  7. You make so many great points here that I read your article a couple of times. Your views are in accordance with my own for the most part. This is great content for your readers.

  8. If your spitting, popping and having trouble keeping the engine turning at idle, you most likely have a slip of the gasket on your t-pipe that connects the carb to the engine. just pull it off, check the gasket.if it's worn at all replace but most often it's fine. Typically a good coat of grease and a refitting of gaskets, with a secure fitting of the part will solve your problems. Keep in mind, if you have been ridding for extended periods of time with the bike being in this condition, you may have damage to your head gaskets as well.

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