'07 950: Popping on Decel

Discussion in 'Crazy-Awesome almost Dakar racers (950/990cc)' started by wiseblood, May 22, 2013.

  1. wiseblood

    wiseblood Hall Monitor

    Dec 29, 2008
    New York City
    Now, let me just start by saying, I do NOT hate the noise. My inner hooligan cracks a smile whenever my exhaust starts singing. :evil

    Mine is a 2007 950 (SMR). Canister and SAS removed. Right-side manifold vacuum ports plugged with screws/loctite. Left side ports have hoses plugged with screws.

    Carbs are mostly stock. Mains have been opened up a bit to 160/165 (F/R). Needle position #3. IMS is three turns out. Exhaust is stock. Motorcycle seems to run great (pulls everywhere), but my fuel economy is laughable (21 mpg / city).

    My question: Is popping on decel an indication of anything bad? What might be the cause of this? How could I stop it, if I wanted to?

    FWIW, I found a very interesting and comprehensive-sounding post on another motorcycle forum:


    Burn Baby Burn

    The last thing I want to address today is the subject of deceleration backfire, or “popping”. This topic generates a lot of concern from inexperienced riders, or even from experienced guys who just hate the noise, so lets take a look at what causes it. But first things first, lets define the issue:

    Deceleration Backfire is caused by fuel burning in the exhaust manifold or header.

    No ifs ands or buts, that’s what causes it. But the bigger question is how does gas get there in the first place, and that’s a bit more complicated. Generally, there are a variety of ways it gets there, and a variety of things that can make the backfiring worse. But there’s a kicker, and something you should understand before we go any farther:

    A motor in perfect tune will exhibit deceleration backfiring.

    Therefore, just because your motor is banging it up, doesn’t mean there’s anything wrong. And consequently:

    Getting rid of the noise means de-tuning your motor.

    Yup. If you’ve just got to eliminate that popping, you’ll have to accept the fact that your motor is going to be forced to run rich to do it, and that isn’t necessarily a good thing. So lets talk about what causes the problem.

    Ok, so you’re riding along at some given rpm, and suddenly you decide to decelerate, and you reduce the amount of throttle. This causes an “overrun” – that is, the motors rpm is turning faster than the fuel provided can support, so the motor begins to spool down. This causes a couple of things to happen.

    First, when you close the throttle, you are also closing the throttle plates. This reduces the air and fuel flowing into the motor, and increases the vacuum (lowers the pressure). This results in less air and fuel in the cylinder during the power stroke, which in turn results in a lower pressure in the combustion chamber. Remember I said earlier, that the A/F mix burns faster in proportion to the pressure applied? Well, when we reduce pressure this way, the mix burns slower. This results in two things happening.

    1. The lower burning fuel generates less heat, and the cooling effect of the non-burning fuel tends to “quench” the flame front, or slow it down even further. Because the mix is burning much slower, the exhaust valve can open before all the fuel is consumed, and the unburnt fuel is ejected into the exhaust.
    2. The engine designers, in order to promote smoother idling and better combustion, retard the spark when the throttle is shut, and this results in the mix being lit later.

    So, now we end up with unburnt fuel in the exhaust, and burning fuel being ejected into the exhaust, and bang! Backfire. In addition, Manufacturers have added a device called a “programmed air injection valve” (Pair Valve) that actually injects some fresh air into the exhaust to help this process along – since fully burning the fuel results in cleaner exhaust. So the backfiring is not only a normal part of the engines operation, it’s also intentionally amplified by the manufacturer! Of course, normally, that massive bazooka pipe hanging on your bike hides most of the noise, but it’s there, even when you can’t hear it.

    So the bottom line, is: That backfiring is perfectly normal and expected. If you’ve just got get rid of it, that’s up to you. You’re entitled to set your motor up the way you want, and your goals are your goals. But don’t refer to it as “fixing” the popping. Rather, the correct way to think of it is “de-tuning a bit to get rid of the popping”.

    There are a few ways you can do this.

    First, use the stock pipe. It will hide the sound, by absorbing it into mass, and masking it with the larger baffle space. Second, you can add more fuel during deceleration. This has the effect of raising the chamber pressure slightly, which burns a little more before the exhaust valve opens. Lastly, you can remove the Pair valve, which reduces the amount of available oxygen in the pipe to burn the unburnt fuel.

    Does this description sound accurate?
  2. DirtyADV

    DirtyADV Long timer

    Jan 13, 2007
    SWEDEN, SkellefteƄ
    Is it just low pops or loud bangs?

    My 950 pops some but had a leak on one of the intake hoses used to sync the carbs and that cause it do have really loud exhaust BANGS when engine braking and it dropped below 4000 rpm.

    Not fully seated carbs or damaged intake boot might cause it also I would guess.

  3. wiseblood

    wiseblood Hall Monitor

    Dec 29, 2008
    New York City
    No loud bangs. Just some moderate burbling when I close the throttle. Happens at any speed, in any gear.
  4. Sumi

    Sumi Long timer

    Mar 30, 2010
    it's normal.
  5. 5 speed

    5 speed Been here awhile

    Sep 12, 2006
    Orlando Florida
    Mine was caused the first time by the carbs not being fully seated in the rubber intake boots. The second time that happened I felt sure it was the same but no. I now think its because I had a problem with one of the vent hoses being kinked and couldnt find where it was exiting and didnt want to take it all apart.

    However, I took it to my dealer and they told me it was too lean in the midrange and rejetted it, among other things larger main, dropped the needle, think they change the pilot jet.

    What that did do is make the bike not idle as well as it used to but, it definitely runs much better in the middle and even cruising on the highway doesnt vibrate as bad. And now, there is no popping on deceleration-none.
  6. Scott Baja

    Scott Baja el semental rojo

    May 27, 2010
    Missoula MT
    Popping is normal as you have described. I think the article you had is correct. My impression is it would be running to rich if no popping. That said, it should not sound like the 4th of July!