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interesting read on decel popping

This is a discussion on interesting read on decel popping within the Mean Streak Performance Upgrades forums, part of the Kawasaki Mean Streak category; 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 ...

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  1. #1
    Supreme Being hawaiian's Avatar
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    interesting read on decel popping

    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.

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  3. #2
    Rising Star halstorm's Avatar
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    nice right up
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  4. #3
    Supreme Being Tom_E's Avatar
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    Pretty much right. The stock pipes mask it. Aftermarket pipes are problematic. Lousy gasket seal (air leaks) during installation is a big issue. They're also big and open and can actually pull air in from the outlet (reversion). The fix is to shut the fuel off during deceleration (which is highest intake vacuum). Or add fuel during deceleration. Too much or too little and the mix is non-combustable. Ever watch Moto GP footage or even Nascar and watch the flames out the pipes when they're off-throttle on corner entry? Lots of fuel and it doesn't bang in the pipe because the mixture is too rich and the fuel won't burn until it sees atmosphere (more air). Why the excess fuel? So the throttle picks up smoothly as you get back into it. An "accelerator pump" can't add fuel quickly enough, you need the extra fuel in the intake manifold/port waiting for the throttle to be opened. A PCIII is the only fuel module that will address this.

    XS11 Yamaha's, for instance, carbs and horrible EPA idle and part throttle, had a circuit added to shut off the idle circuits at high manifold vacuum. Deceleration manifold vacuum is much higher than idle vacuum. It worked but gave a bad hesitation when rolling back into the throttle. A company actually made an add on accelerator pump to help fix them. Disableing the shut off (and richening the idle) worked better.
    Last edited by Tom_E; 12-05-08 at 02:29 PM.

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    Rising Star carvedup's Avatar
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    Good discussion, hawaiian.

    I'd like to add that with a PCIII you maybe can tune out decel pops without effecting the performance too much. I accomplished this by leaning out settings below 20% throttle and 3500 rpm. I did this recently to increase the fuel range while cruising. You may have noticed my post. I think others here have done the same thing. So far I have taken 15 units from that range for economy range. A bi-product was that it all but eliminated any occasional pop. This hasn't effect the throttle position and power range I use for performance.

    This is what the spark plugs look like. What do you think..... Still a little rich maybe?

    '08 - Fire Candy Red
    ================
    Vance & Hines Big Shot pipes
    "Halfway to Thunder" air mod
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    Site Elder zootech's Avatar
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    Good explanation, though it may be of interest to add that the term "back-fire" is used incorrectly most of the time, including in your write-up. A back-fire condition occurs in the intake, as was common with older carb'd vehicles that were out of tune. An explosion of unburnt fuel in a vehicle's exhaust, on the other hand, is properly known as after fire.
    ~Bikeless~

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    Up-And Comer jagr's Avatar
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    This is how we "fix" it on 2nd gen SV650s.
    Not sure if this would apply to other FI bikes.

    To eliminate decel popping:
    Plug off the air going to the exhaust ports via PAIR valve. Go to the bottom right side of the air box and find the T junction with the apox 1/2"inch black hose which leads to the front and back cylinder and PAIR valve. You need to plug this connection. So pull it off at the T and plug it with a 3/8" inch rubber plug that you can find at any auto parts or hardware store. It works perfectly and no more popping at all.
    No other mapping or re-tuning is needed of you stick with a stock Suzuki air filter.

    03 Kawi Z1000 (rebuilding)
    06 Zuki SV650S

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    Supreme Being Tom_E's Avatar
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    Quote Originally Posted by carvedup View Post
    This is what the spark plugs look like. What do you think..... Still a little rich maybe?
    Unless something is grossly wrong, about all you can tell looking at the plug color in a picture like that is that the plug's heat range seems reasonable.

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    Rising Star carvedup's Avatar
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    Tom, I'm surprised that I could lean the mixture that much and not get into a "too lean" condition. I figure it is something like 13% less fuel with the changes. I don't think I will adjust the map anymore. I'm happy with the performance I have now. Before I was getting between 32 to 38 mpg. Now it is between 32 and 44 mpg.

    The next thing I'm planning to do is installing Race Tech fork springs. The guy who is doing it has been to their tuning school. He is letting me assist which I like. I hope to get it done next week. Weather and time permitting.
    '08 - Fire Candy Red
    ================
    Vance & Hines Big Shot pipes
    "Halfway to Thunder" air mod
    PCIII
    Race Tech fork springs
    Custom made sissy bar
    Custom made windscreen
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    Supreme Being Tom_E's Avatar
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    Quote Originally Posted by carvedup View Post
    Tom, I'm surprised that I could lean the mixture that much and not get into a "too lean" condition. I figure it is something like 13% less fuel with the changes.
    Do you have the "accelerator pump" installed?

  11. #10
    Supreme Being troop's Avatar
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    Quote Originally Posted by Tom_E View Post
    Do you have the "accelerator pump" installed?
    I enabled my accel pump feature with settings provided to me by Jamie at Fuel Moto and couldn't tell any difference. I believe my settings were 75% sensitivity, 15% fuel and 20 revolutions. Certainly would want this disabled when making a map or doing adjustments. I disabled it.....
    ** Gone But Never Forgotten **
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    *Now a H-D bagger owner with a 103"/cams/head work build in progress *







    See more at : www.cruisercustomizing.com/troop

  12. #11
    Rising Star carvedup's Avatar
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    Quote Originally Posted by Tom_E View Post
    Do you have the "accelerator pump" installed?
    No I don't understand how to set it. Do you have any input on setting up the accelerator pump?
    '08 - Fire Candy Red
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    "Halfway to Thunder" air mod
    PCIII
    Race Tech fork springs
    Custom made sissy bar
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    Supreme Being john56's Avatar
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    so if the throttle is closed why is there gas coming in.
    My 1600 with 3" big shots does not pop, maybe I cant hear it. It did pop with bad gaskets though.

  14. #13
    Supreme Being Tom_E's Avatar
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    Quote Originally Posted by carvedup View Post
    No I don't understand how to set it. Do you have any input on setting up the accelerator pump?
    You have to download and install it. DynoJet's installation guide has starting reccomendations. The richer your mapping the less you need it. You can only load it into the USB PCIII, not the older serial PCIII. I've got my cruise settings as lean as I can run without losing throttle response. Mileage is a consistent 38-41. It will run leaner, there's more mileage to be had, but throttle response gets lousy. That's where the accelerator pump comes in. Unfortunately I have the old serial port PCIII, so I'm stuck. The "pump" is very adjustable, how easily it's turned on, how much it adds and how long it runs. It basically just richens the mixture briefly. In Troop's example, when the "pump" is triggered, it adds fuel for 20 engine revolutions. At 3,000 rpm this is 4/10 of a second. One more tuning aspect, but it will allow leaner cruise mixtures while retaining good throttle response.

    The download and installation guide are on DynoJets site: http://www.powercommander.com/powerc...downloads.aspx
    Last edited by Tom_E; 12-08-08 at 08:10 AM.

  15. #14
    Rising Star carvedup's Avatar
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    Seems to be a real nice feature. I downloaded it, but I'll have to wait a while to load it and try it out. I'll play with it some and get back with any results.
    '08 - Fire Candy Red
    ================
    Vance & Hines Big Shot pipes
    "Halfway to Thunder" air mod
    PCIII
    Race Tech fork springs
    Custom made sissy bar
    Custom made windscreen
    Custom graphics

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