Motorcycle Club » Star 1100 Classic & Custom

popping on deceleration

48 posts from 16 voices
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  1. wzdartemon

    Any setting help is good,bike has been back and forward to bike shop for popping/backfiring on deceleration,just cant get rid of it.,running pro kurakyn hypercharger and cobra pro hp 2 into 1 exhaust ,was previosly running with v+h slippons,still the same issue,bike shop having issue resolving this.any common jetting size pms etc for this issue would be appreciated.

  2. wzdartan

    Is the A.I.S. removed? It sounds like a lean condition. What do your plugs look like?

  3. bkman

    I would PM Mick. I think he has this same intake system and he seems happy with it. I've read that hyper-chargers are sometimes difficult to tune, but that may be untrue. I don't have one so I don't know.

    Check your vacuum hoses and make sure they are not leaking anywhere.

  4. JG6

    I would disable or remove the a.i.s. I disabled mine and no popping at all decel.

  5. quicgmicg

    All the above suggestions are good. I've noticed several things with my set up.

    ONE: Weather seems to have an effect on backfiring. When it rains here in Tucson, it gets humid and my backfiring seems to decrease.

    TWO: Altitude seems to have some effect as well. I'm still trying to figure what altitudes are the bad ones. I generally ride between 2,000' and 6,000'.

    THREE: No matter what I do, some times I get some backfiring, sometimes I don't. I'm going to keep tweaking and I'll let you all know what I find out.

    FOUR: The Guys at Bikers Bay here is Tucson have got me pretty well dialed in and backfiring has decreased 70% before I had them re-jet.

    .... and really, backfiring is caused when unburned fuel in your exhaust collects in the exhaust pipe and ignites. The only real solution is to figure out a way to clear out all of the exhaust gas or add mufflers to cut down on the sound....or just live with an occasional backfire... I think it's kind of bad a**.

  6. wzdartan

    From all I have read. quicgmicg is exactly correct. Bottom line is, backfiring through the exhaust is caused by a build up of raw unburnt fuel in the exhaust. The only reasons raw fuel would be accumulating in the exhaust is because the bike is running too rich ,or too lean. Both situations will result in a incomplete burn of the fuel within the combustion chamber. The perfect mixture of oxygen and any fuel (air to fuel ratio) creates the most complete burn of the fuel. Any "leftover" fuel will ultimatley be dumped into the exhaust (bad for emissions). The bikes are set up to run lean from the factory, to lessen the amount of raw fuel being dumped into the exhaust. The A.I.S. system on our bikes pumps fresh oxygen into the exhaust to help ignite any remaining fuel before it is ejected out of the exhaust (good for emissions). Humidity or elevation both will make a difference in the air fuel ratio (high elevation= less oxygen-high humidity=less oxygen), so it would make sense that the variances in these might effect the way a motor runs. The above rant may help to explain why backfiring through the exhaust is a matter of being able to tweak the tuning of the motor by virtue of rejetting and adjusting the pms on the carbs. No raw fuel in exhaust=no backfire. It is also an explanation as to why sometimes disconnecting the AIS will not help with backfiring, and in severe situations may even make for more violent backfires. After all, it takes a good afr to burn the fuel thouroughly within the exhaust, just as it does within the combustion chamber.. If all of the oxygen was used to ignite the fuel in the combustion chamber, more must be introduced into the exhaust to completely burn any fuel that ended up there. The true secret to getting a bike to run strong without backfire, is to tweak the carbs so the afr is as close to perfect as possible throughout the entire throttle range. At least that is the way I understand it. Does this sound right to you guys? I am no "crack shot performance tuner" by any means, but I try to learn as much as I can by reading posts from guys that are.

  7. wzdartemon

    Is the A.I.S. removed? It sounds like a lean condition. What do your plugs look like?

    AIS removed, plugs centre is grey ro the electrode,the out part of the plug is black
    Also I had bike at shop several times to no avail,
    Could any let me know if the jetting for a kurakyn hypercharger with cobra 2 into 1 exhaust should be 120/122.5 main jet,thats what the shop put in but when I go to they reccomend larger jets like 150 etc???

  8. bkman

    I know the guys over at Slone recommend larger jets than some others (like Maxair), but it seems to me that popping on deceleration points to the PILOT circuit of the carbs, not the main jets. That being the case, I think the pilot mixture is the thing you want to focus on. Please take my advice for what its worth - I am only began fooling around with this stuff about 18 months ago. If you know that all your vacuum hoses are on tight, and that there are no other possible air leaks (downstream of the carbs), then I think you need to start playing with the PMS - try turning them one way first and see if things get worse or better and make sure your carbs are balanced because I DID have some minor popping before my carbs were synchronized.

    For what its worth, I have the same exhaust as you, but a different intake (Maxair) and I have no audible popping at all (with AIS removed and properly plugged). What pilot jets and mixture screws are you running? I am using the Max-Mix screws (the new ones) and the pilot jets that I got with my kit, but Barons also makes good pilot screws (I have been told). If you (or anyone before you) did any carb modifications, you might want to double-check the pilot screws. They are easy to damage if they are closed too tightly, and there is a tiny o-ring and spring on the end of them that can be damaged or lost. Either way, it can really mess up your pilot mixture if these screws are not perfect. I run my PMSes at about 2.75 - 3 turns out (the rear jug PMS is set a little leaner than the front - it just seems happier that way) and I get no popping. I usually do not need my choke at all when I first start the bike, which probably means I am a little too rich, but that is okay with me. The exhaust doesn't stink too much so I know I am not overly rich. Also, my plugs look normal, perhaps slightly darker than normal.

    I do think that my setup leans out at 3/4 to WOT, which is also okay with me because I don't ride for long periods of time in that throttle range. A little bit leaner in the top end might even help performance a little (or maybe not - I am not sure). If you go with larger mains, your mixture would probably be more consistent across the throttle range, but I have noticed no negative effects with my current setup so I choose to leave well-enough alone.

    By the way, the outside part of my plugs are black also - I don't think this is something to worry about - you want to look at the center electrode and ceramic tower when you inspect your plugs.

  9. wzdartan

    Grey plug towers lead me to beleive the bike is running lean. I think it is very difficult to find a shop that really knows or cares what they are doing when it comes to tuning a bike. I agree with bkman in that I would start with the pms adjustment. I would screw the pms in untill they were LIGHTLY seated on the "o" rings, and make a note as to how many turns they are currently set at.
    I would then reset them to where they originally were, then begin to back them out a quarter turn at a time. Take the bike out and see how it runs after each adjustment. I would try this untill I reached three and one half turns out. I beleive this is about as far as I would want to go with the screws.
    If the way the bike is running, and the way the plugs look after going 3 1/2 turns out still indicate a lean condition, you may have to add a shim to your needles and start the whole process over. As bkman said "you can take this for what it is worth". I am not a mechanic nor am I a performance tuner. This is how I set the carbs on my four wheelers and such. The knowledge base at "sloneservices" is an abundant source of information compiled by tens of thousands of V Star owners and tuners. It is worth anyones time to read it thoroughly. It may help you to decide what you want to do. Good luck!

  10. fupar

    I don't think my advice will be of any help to you, because my system is pretty much stock, with the exception of the V & H pipes. My AIS is still hooked up, I haven't changed the jets, etc.

    Mine was backfiring a lot on decel.

    I took the bike in about 3 weeks ago to have the 4000 mile service done. Part of the service was to sync the carbs. Since then, I have had no backfiring at all.

    Couldn't hurt to check the carbs.

    Just my 2¢.

  11. bkman

    If the way the bike is running, and the way the plugs look after going 3 1/2 turns out still indicate a lean condition, you may have to add a shim to your needles and start the whole process over.
    The only thing I would say in response to this is that I think I'd increase the size of the pilot jets before I messed with the mains - that is, unless there is some reason to believe that the bike is running way too lean over the rest of the throttle range.

  12. quicgmicg

    VSTAR DAN...

    Thanks for explaining it so we could understand it!

  13. Nagona

    Mine was doing the same thing until I removed the AIS and it stopped.

  14. wzdartemon

    MEMBERS <THANK YOU ALL VERY MUCH,I really aprreciate this and will try the recommendations 1 by 1 ,There seems a lot to go thru but Im finding adjusting the PMS to 3.5 turns a bit better but the acceleration has seemed to be less responsive.Im going back to the start and recheching the sync and will take on board your vast expertise and rule out item by item
    Will let you all know in a day or two how it went,My hat is of to you blokes!!

  15. bkman

    Good luck to you. I think that carb synchronization is the best place to start - a couple of us mentioned that. Don't over think it. You may find that it is all good once you sync the carbs.

  16. wzdartan

    This is some pretty good info

  17. wzdartemon

    Ok here is the hard facts
    checked syn,OK
    Adjusted PMS to 3 1/4 to 31/2 turns.A little better .
    Running 120/122.5 jets.
    Still get popping at lower rpm at deceleration,ie going down a hill at 5th gear.
    Well I took it down to the bike shop out of frustration and dynoed the bike,
    Stats are fantastic 67HP and no popping on the dyno at all?????Torque curve in relation to HP perfect.I stayed there with the dudes to make sure.
    Only pops during decel and the only difference is that the dyno has a constant load to the rear tyre.
    So if Im coasting down a hill or decelerating and getting down to 2000 rpms or lower the bike will pop or backfire,
    It doesnt pop if I keep the revs up or use the brakes instead of engine breaking.
    The bike shop has slugged me to much over the months ,but again they are at a loss????

  18. wzdartan

    Ya know, my bike "pops" a bit on decel also. I don't classify this "popping" as the same as true backfiring. I think that one should expect some measure of gurgle and pop on decel when running exhaust systems that are designed to allow the sound of the motor to pass through them.
    I'll will get a small backfire every once in a while, but it usually happens when the bike is cold. I would most likely be able to eliminate this with some additional carb adjustment, but I'm going to wait to do that. I'll try to get everything as perfect as possible when I install my pods.
    I have friends with Harleys that are both carb. and f.i. machines, their bikes will also pop a bit sometimes. I still beleive it is a matter of acheiving the correct afr throughout the throttle range to eliminate the majority of this condition. Trying to do this can be tricky and time consuming. Because the variables effecting the afr through the carbs are so great, I can see where this could be a frustrating task. Just out of curiosity, when the bike was on the dyno, did they check the afr? Did you clean or replace the plugs and recheck the colour after adjusting the pms?
    Your dyno numbers do sound good, and if the bike is running really well I wouldn't get too bent over a small bit of popping and an occasional small backfire. It sounds like you are close, and a small change somewhere will most likely cure the situation. You will just have to play with it untill you discover exatly where that change needs to be made.
    I would most likely look at the pilots or needle shims next. Pilots and pms will have some on the afr throughout the entire throttle range with the largest effect near idle. Needles will have the largest affect the afr from about 1/4 to 3/4 throttle. Mains will have the largest affect the afr from about 1/2 to w.o.t. All of these items will overlap or have periods ov overlap throughout the entire throttle range. But knowing where they have the greatest impact could give you a clue as to where changes need to be made. Good luck!

  19. wzdartemon

    Thanks wzdartan, You could have a point there ,it on occurs when bike is hot .
    Will try larger pilots next week,will need to source,also bike shop has taken an interest chasing up the issue to other mechanics to which they'll get back to me,definetly better responsiveness and a bit less poping/backfiring after playing with the PMS,bike is new, only 1300 km on it.Plugs etc ok

  20. wzdartan

    Hey! I just thought I would let you know that I was not correct in the number of turns you can go out to with the pms screw. I just found out that the pms screw can be turned out 4 1/2 turns before you have to change the pilot jet size. Sorry for the incorrect info. Maybe that will get you where you need to be withiut changing jets.


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