Dunstalls on my r90/6

Discussion in 'Airheads' started by Buffel, Mar 25, 2012.

  1. Buffel

    Buffel Been here awhile

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    I just installed dunstall replica pipes on my r90/6, but now it stumbles more than often, not in a certain rpm range, except at idle. Why does it do this? do i need bigger jets because i have an aftermarket exhaust? any help is welcome:ear
    #1
  2. OldPete

    OldPete Be aware

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    this mod on many a bike causes a drop in the lower mid-range, a hole in the torque curve.

    i put slip-ons on a r100rt and the same thing happened but not too bad, in 1985.

    you should state what kind of carbs are on the bike... bings?

    with mikuni carbs the slide cut-away can be increased iirc.

    do a google on this yet?
    #2
  3. Buffel

    Buffel Been here awhile

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    yes googled it, couldnt find that much info:/ but the rest of my bike is stock
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  4. OldPete

    OldPete Be aware

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    from this post it was already having some running issues and the dunstalls just aggravated them.

    the bing cv 32mm carbs in good shape? not leaking air at the manifolds, are the diaphrams OK, are they balanced? whole bunch of stuff...
    #4
  5. Buffel

    Buffel Been here awhile

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    Ill do some work on it this weekend, valve clearance, ignition points, balancing cabs, hope this will solve it
    #5
  6. boxerboy81

    boxerboy81 Stay Horizontal

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    What needle and main jets are you running? The main is simple and cheap to swap one size bigger.
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  7. Buffel

    Buffel Been here awhile

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    All stock 150 main
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  8. boxerboy81

    boxerboy81 Stay Horizontal

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    Try 155, see what happens. They're only a few $.
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  9. cycleman2

    cycleman2 Been here awhile

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    If the stumble is just off idle it has something to do with the idle or less than 1/4 throttle circuit. Could be as simple as the idle air adjusting screw not being set properly, or one of the small holes in this circuit being partially plugged.

    With carbs you have to figure out where in the throttle range the problem is and then deal with that part of the circuit that controls that part of the throttle.
    #9
  10. mykill

    mykill odd

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    Exactly! You need to focus on the throttle position when you get the stumble. Mess with your idle mixture first, but after you know everything else is set to spec. A lean idle will cause a stumble rolling on and possible popping on decel. Hopefully you won't be getting into different needle jets and main jets or combinations thereof.
    #10
  11. supershaft

    supershaft because I can

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    Freer flowing exhaust rarely calls for bigger mains although that is what they often get. It's cheap and easy! It usually robs hp and doesn't help the actual problem. Lose/lose. Freer flowing exhaust almost always actually leans out just off idle. With our Bings, that is mostly cured with richer needle jets. If it's got 266's, 268's normally do it. Sometimes it will take 270's and so on. Chances are you need smaller mains for more power and better economy but always remember that best power and engine efficiency are real close to best meltdown. That is exactly why so many aren't there! :D If it was easy it wouldn't be so much fun! Good luck!
    #11
  12. Buffel

    Buffel Been here awhile

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    It does pop a little when i close the gas, so a bigger needle jet? Which One should i get? My carbs are all stock
    #12
  13. boxerboy81

    boxerboy81 Stay Horizontal

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    Stock 90/6 needle jet is 268.
    Would you blokes go a bigger needle jet or up the needle height first?
    #13
  14. Beemerboff

    Beemerboff Long timer

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    Float level matters - Snowbums site has the information on how to set the actual depth in the bowl, and getting it right is important

    The later carbs have a different, stiffer spring above the slides, so order them under the latter number

    Try them , if it makes things too rich you will probably be able to drop the needle a notch and still be able to richen the mixture up in transition, where you need it.

    I put a Dunstall Decibell on my 900 cc 75/7 with a 2 in 1, had a similar problem and the springs and dropping the needle cured it, so I am not farting in the dark --------.
    #14
  15. Buffel

    Buffel Been here awhile

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    How do i change the float level? I found what the correct amount of petrol that should be in the bowl but dont actually know how to change it:/ and where is the needle in the carb is located? So many questions... Haha
    #15
  16. supershaft

    supershaft because I can

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    Popping on decel is fixed by turning out the mixture screw. The needle jet has very little to do with it and the jet needle none. Some setups run best popping under decel.

    Raising the jet needle largely effects up above 1/8th throttle. The needle jets largely effect JUST off idle. Which one to do? On bikes that have the jet needle on the second position it is a toss up and/or a combo. On bikes that come in the third position? I have never seen any run best on the forth position for being too rich at 1/3 throttle. They always rather have the needle jet adjusted. Good luck!
    #16
  17. mykill

    mykill odd

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    Float level is changed by CAREFULLY bending the tab on the float assembly that touches the float needle (where the gas comes in from the tank),

    The Needle jet is in the piston, points down into the jet needle, atomizer and main jet assembly. Changing needle positions differs between Bing models.
    #17
  18. Buffel

    Buffel Been here awhile

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    i messed with the idle screws, made it richer, stumbling is gone now. but i had to put the idle stops up or else it would die:/ and it stills pops a little on decel, wich i dont really mind:wink:
    #18