r100gs: mikuni carbs swapped in for the 32mm bings?

Discussion in 'Airheads' started by crowflies, Dec 21, 2009.

  1. crowflies

    crowflies n00b

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    just wondering if there is anyone out there who has successfully replaced their stock bings with an aftermarket carb. my carbs are a mess on the bike i'm (almost done!) building, and i'm thinking it might be easier and better and maybe even cheaper to replace them rather than rebuild them. anyone use mikuni flat slides on their airhead? would love to hear about the details of the swap. and the jetting i'm running a supertrapp full stainless, motor is stock.
    #1
  2. bgoodsoil

    bgoodsoil Dare to be Stupid

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    do a search for mikuni in this thread. You'll find plenty. Stan at Rocky Point makes a prejetted kit with intake boots and pre-made cables. A no-brainer 10 minute bolt up.
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  3. mykill

    mykill odd

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    Flatslides will be very abrupt in their power delivery and setting up can be a challenge. Not sure if they are a good fit for a stock engine. VM's are fine but even pre-jetted carbs can use some tweaking, that is where the obsession begins.
    How do you want to use the bike?
    For all around riding and traveling I'm a Bing fan. Mikunis work well but the bike loses some of its sweetness. Bings seem to have a lot of jetting latitude and are very forgiving.
    Raised compression, head porting, cam, lightened flywheel and flatslides will make a snarling beast (relatively) that you will never be able to realax on.
    Personal preference!
    #3
  4. wirewrkr

    wirewrkr the thread-killer

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    What he said, very well put and right on the mark.
    NO such thing as a pre jetted kit that works for everyone.
    Robert
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  5. Boxer Metal

    Boxer Metal Mad Scientist

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    The other option is all of the parts from a European R100GS. They ran 40mm Bings with the same cylinder heads. Works amazing.
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  6. wirewrkr

    wirewrkr the thread-killer

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    Yeah, and I can still get the manifolds right from BMW at a reasonable price.
    Robert
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  7. mykill

    mykill odd

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    +1
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  8. walkingbear

    walkingbear the road of life is full of twists and turns.

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    ok, I don't get it . It's more gas than it can burn? My understanding
    that you needed bigger heads to handle the 40carbs.

    Educate me!
    #8
  9. datchew

    datchew Don't buy from Brad

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    I think (correct me if i'm way off here) that the 40mm carbs still only provide as much fuel as the motor needs depending on how you treat the throttle. The difference is that they have the potential to deliver more fuel when you really want it.
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  10. bpeckm

    bpeckm Grin!

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    The principle behind the CV carbs (constant velocity) is that the slide/needle rises, based on the pressure differences ("vacuum") of the air running through it, which is governed by the throttle plate, which is what is actually moved by your right hand. The CV's are in fact quite forgiving of things such as altitude (i.e. variations in air density), used in a wide variety of conditions on a wide variety of bikes....

    The air volume that is drawn into the combustion chamber is based on two things (to put it in simple terms): the swept volume (cc's / displacement) and the speed of the engine. So yes, the CV's will "adapt" by opening the slide further for a higher displacement engine. The biggest difference between the 32mm and the 40mm Bing CV's is really only at high throttle/high speeds, when the 40mm opening can physically pass more air. The total available area of the 40mm is not being used until the slide is raised up by high engine speeds and high power demand ("as needed" by the air pressure differential. Carb theory is that a smaller carb does a better job for torque and low rpm's, the larger carbs give the high-end horsepower.

    If you look at the history of the BMW's with Bing CV's, you will find that the 32mm was used for the R65, the R75, the R80 and even the R100.... quite a range of displacements. Of course they are jetted differently, and there are some minor variations of the carbs over the years, but I think it says volumes about the use-ability of the type.

    :D
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  11. walkingbear

    walkingbear the road of life is full of twists and turns.

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    Then, the advantages of using a 40mm carb is for the avail extra potential horsepower at the high end of the RPM range. What you might lose is the greater torque value at the low / mid range.
    #11
  12. Infracaninophile

    Infracaninophile Finding My Way..

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    Great post. I guess it depends a lot on what speed you tend to run your engine. I try to keep mine in the 3500-4500 range whenever possible. I almost never really rev it up so based on your comments above I would not benefit from bigger carbs.

    Back in the day with muscle cars I had a Mopar with a 340 ci in it. I put a mid-sized Holley 540 (IIRC) on it. Friends with basically the same motor were putting much larger carbs on there. Their cars did not appear to be faster so I never understood the appeal. IMHO, bigger is not always better.

    t.
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  13. walkingbear

    walkingbear the road of life is full of twists and turns.

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    I spoke to my mechanic who trained in Germany.
    He said that 40's in europe ran on big heads not small heads
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  14. ChromeSux

    ChromeSux Un-plated and Unscrewed

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    as asked above, correct me if i am mistaken but dont the smaller bore carbs help the torque in the lower rpm range and you give some of that up by going to the bigger bore carb which may help the top rpm range.

    bigger bore carb = top range of rpm power
    smaller bore carb = lower range rpm torque
    #14
  15. opposedcyljunkie

    opposedcyljunkie Heavyweight Boxer

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    I don't think so... I believe US and Euro models ran on the same heads.
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  16. mykill

    mykill odd

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    Euro GS's used the small valve heads with a manifold that adapts the 40mm carbs.
    Pt#11121257396 might be the part number. This was from a Eurotech receipt from back in 1996, only$11.50 each! A&S shows $41.00 now. Description only says Intake Manifold, but I'm pretty sure these are it, check to be sure.
    #16
  17. beemerjim

    beemerjim Been here awhile

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    I had an 82 r100 that I put Mikuni flat slides on, and after I got them jetted right (they had been on another r100 and only needed a little tweaking) the bike ran fine. I remember them to be much more responsive than the Bings,
    but not much difference on the top end, and the gas mileage went from about 45 mpg to under 30 mpg. The thing would really jump when I snapped the throttle open though.
    The slides eventually wore, and made them run lean, and I took them off.
    #17
  18. bereahorn

    bereahorn Long timer

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    PM sent
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  19. Drumboy

    Drumboy Been here awhile

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    Bumping an old thread...

    I picked up a GSPD that has Mikunis on it and it runs great but there's not much clearance between the bottom of the bowl and the brake lever. When wearing my riding boots (Icon Patrol) have to be cognizant of the angle needed to get my foot above the brake lever.
    #19