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Discussion in 'Technical and Troubleshooting Torque' started by Wolfye, May 7, 2008.

  1. I have a Yamaha XVS1100 that I bought new a few months ago. Twice now on starting her up she has backfired with flames coming out of the exhaust pipe. It was so loud that it made my ears ring for a few minutes afterwards.

    I guess what I want to know is:

    1. Is this normal?
    2. Is it harmful to my bike?
    3. Is there anything that I am doing wrong that is making it do that?

    Thanks in advance.
  2. Are the exhausts and carburetion standard or aftermarket?

    What's your starting procedure?

    Loud bangs from the exhaust tend to be a symptom of a build-up of unburned fuel and vapour in the pipes, which then ignites and explodes. This could be happening if stat-up involves lots of churning the engine over with full choke on.

    Try using less choke as a first step.

    It's unlikely to damage the bike, although I've seen extreme cases blow car exhausts apart. I was deliberately provoking that though :twisted: .
  3. Thanks for your response PatB

    I have aftermarket exhausts (Vance & Hines)

    The two times it has happened I was choking it and it probably didn't need any choke.

    I'll see how I go just using a little throttle to warm her up instead once she turns over to see how that goes. She does warm up pretty quick in any case.
  4. A V with non-standard pipes is very likely to frap and bang a bit on the overrun, with little flames from the tailpipes. That's what makes them so much fun to follow downhill in the dark :grin: .

    As for starting, let us know how you go with reduced choke use. Go easy on blipping the throttle too. That can squirt excessive quantities of raw fuel in (depending on carb design). A moderate, steady setting is generally a better bet for starting.
  5. The backfiring and popping on the VStars is common, and becomes noticable mostly if you mod the original pipes or put on aftermarket exhaust.

    It's the AIS on the bike (Air induction system) which is the cause and is supposed to burn off any unburnt gases in the exhaust. Once you open up the pipes you get popping and backfiring. It's not a problem you need worry about - unless the sound bothers you.

    You can however either remove the AIS totally or just disable it (takes a few minutes).

    There's a good guide on how to do it here
  6. Thanks for the suggestions! I've changed a few things about the way I start and stop my bike and (for now) she doesn't backfire on start like she has been.

    1. When I stop the bike, I let her idle for a few seconds before I turn her off.
    2. When I start the bike, I don't use any choke at all in the first instance and then mild throttle. If that doesn't work I give her a little choke.

    The popping that I get when changing gears doesn't concern me that much. It was mostly the backfire as I start my bike at around 7am and don't really want to upset the neighbours. Though when she's out of warranty I might look at the AIS mods :)
  7. Oh, yeah.

    The Icicle Run last year was such wicked fun, watching little blue-orange flickers in the glasspack exhausts of the other bikes, every time we changed gears. :)
  8. Agreed, it does look grand. There was a Yellow 250 Hyosung in the fast group with a megacycle exhaust, running rich, every deceleration it was popping and farting
  9. Yeah, I was right behind that guy on a black VTR250 w/ megacycle. :)
  10. Usually, exhaust popping after a muffler replacement is due to leanness.

    A frequent cause is an air leak somewhere in the exhaust system, anywhere from the headers in the exhaust port to the muffler join on the header(s).

    As the bike decellerates on a closed throttle the engine will actually try to draw air in via the exhaust port or any nearby leakage point, due to the exhaust pressure waves leaving a partial vacuum behind them, for some small part of the combustion cycle.

    As this oxygen-rich fresh air mixes with unburnt fuel in the exhaust you get the little explosions. Some people like this, but knowing what it means (a poorly tuned or maintained engine at the very least) I don't.

    Normally, leaning off an engine via the pilot mixture screw does not result in as much popping (or any) as when the exhaust seals are in poor condition. Even if you didn't remove the header pipes, the effort in removing the average stock muffler is usually enough to "tweak" the seals on the header pipe(s).


    Trevor G
  11. Rubbish. Popping under deceleration/overrun is normal on any open, unrestrictive exhaust system.
    It is caused by the idle circuit providing fuel for combustion instead of teh main circuit, when teh engine is under compression load and usually spinning at a fair rpm. So, teh carburettor/s are providing a mixture that is too lean for full combustion, and the combustion continues as the gases enter the exhaust system. As they get close to teh exhaust outlet, the abundance of oxygen allows combustion to finish, causing teh pop and sometimes flames.
    It can also be caused by being far too ruch on teh over run, but that is hard to achieve with carburettors (but easy to do with fuel injection, which is why you will see race cars belching black smoke and flames on teh overrun.
    An exhaust leak will make the popping worse as it allows oxygen into the system further upstream of teh exhaust outlet, allowing combustion to be completed further up the exhaust system.
    Seriously, there is so much bullshit shoveled on this board sometimes, by people who should know better.

    Regards, Andrew.
  12. And this is effectively exactly what the AIS on the VStars is doing, by introducing oxygen into the exhaust system and why disabling or removing it altogether is so popular on this bike ;)

  13. That's what they say on the american Mikuni site, but I don't agree that it's "normal" in the sense that it is unavoidable.

    We recently repainted the exhaust pipes on the VTR. It was just black paint (with a clear coat), but after we carefully replaced the system, we had exhaust popping on over-run.

    Did we adjust the mixture to get rid of it?

    Didn't give it a thought.

    Since black paint with a clear top coat is not normally associated with exhaust popping, even with a staintune fitted, we went to work. After a 3 second round-table conference we decided that the most likely reason would be: an air leak.

    Potential places? The exhaust port seals - we hadn't replaced them, so we made sure there was no looseness there.

    There were two other possibilities - the clamp on the pipe join for the rear cylinder, and the clamp on the pipe join to the staintune. A slight readjustment of the clamp so that the join(t) was no longer visible and accessible to the outside air was all it took.

    Quite surprising, really.

    I sort of agree, but I cannot find my manuals from the 70s which cover some of these things better.

    Reading the web it seems that some people are having a bet each way - some say the fuel is too rich, others say it is too lean.

    What I DO know is that the outside air can only get into the system and mix with the unburnt exhaust gases and produce a bang because it gets sucked in (like some netriders, you and I agree on that!) through those miniscule gaps in the exhaust system, by the partial vacuum which immediately follows each exhaust pressure wave or pulse.

    A normal system with backpressure will confuse the pressure wave enough so that the partial vacuum is not so present. A low restriction system emphasises the pressure wave and also creates a better opportunity to draw air in.

    Seal it properly and it won't/shouldn't happen via the exhaust system.


    Trevor G
  14. backfiring and whining noise on new ninja 250cc

    I'm not sure whether I am supposed to start a new thread on back firing or do i just add to this one.

    I have a Ninja 250, which is 4 months old. In the morning on starting up lets out a loud bang. It has had its first service. I have not adjusted or added anything. I haven;t checked for flames, might have to do so and have a piece of toast handy. This bike needs a full choke or it won't start. It doesn't do it any other time.

    The other thing is a whining noise when I put it in the garage, it whines for about 5 minutes???
  15. Re: backfiring and whining noise on new ninja 250cc

    My bike made lots of loud bangs when the plugs were shot. As others have said, its unburnt fuel entering the exhaust and igniting. I couldn't see any flames. Depends on the structure of the exhaust i guess.

    On your bike, i'd say either the plugs are worn, or the bike is very cold and needs some choke as its too lean to ignite otherwise.

  16. And the problem with that is? :grin:

    To add to the popping noise, my lil twin does it when decelerating (usually once revs drop below 5,000 rpm) now that it has an after market slip-on pipe. I find myself closing the throttle in 5th a lot more often now :grin:. I'm no expert but I don't think there's anything wrong with it doing that. Again, I have no definitive evidence but I don't think its running lean considering I can always put my hand on the exhaust pipe (carbon fibre) as soon as I've stopped riding and it is only warm near the base, never hot (and cool near the tip).