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  1. #1

    Carb Vs Throttle Body Size

    Something to remember when comparing carb sizes to throttle body sizes.. Carb diameters are measured at the spigot end and are always larger there than they are at the venturi area.. Mikuni 44s are only 40 mm at the venturi, 42s are 36 at the venturi, 40s are 34 at the venturi..
    Throttle bodies do not have a venturi area and are straight the entire way thru.. 40s are really 40s and so forth..
    This is probably why Yamaha put 40mm throttle bodies on the gp1300. Sounds small, but they are the same diameter as the venturi is on mikuni 44s, and probably have almost identical airflow volumes because of that, and if i were to guess, a 40 mm throttle body will outflow a 44 mm carb, because even though they have the same throat diameter, a carb also has a booster in the way to alter air flow also. All of that considered, a 40 mm t-body probably flows as much as a 46mm carb..
    At this formula, 4-6mm difference, any 42 - 44 mm bored throttle bodies are in essence the same as 48-52mm carbs...
    This is where my thinking comes from that bored throttle bodies will be overkill for all but the big bore engine.. Maybe im wrong, only testing will tell..
    Ross


  2. #2
    flyboy's Avatar
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    put 48 Novi's on a 1300 and watch it FLY!!

  3. #3

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    Ross have you ever saw the thoat of a 48mm blackjack carb?

    They are almost a true 48mm all the way through with nothing hanging in the center. There is no way a 44mm throttle body will even come close to the flow of a 48mm blackjack.

    Here is a pic, take out the nitrous fogger on the front and all you have is the butterfly plate.
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  4. #4
    Chrisrokc's Avatar
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    Holycrap! Not knocking you skill or anything Ross but that looks open to me and way bigger than the stock openings on the EFI.


    What gains do people see running those on a 1300? I am curious because I would like to know what the gain could be by boring out the stocks bigger.

  5. #5
    philip_gpr's Avatar
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    Don't forget the 43 mm TBs that Eric tested had a huge gain in the mid range.

    So let's not debate if bigger is better anymore. We know bigger TBs work.

    What we don't know is how big is too big. So should we be focussing on boring out the Yamaha bodies (limit would be 43 or 44 mm) or should we simply replace them with a TB even bigger (48 or 50)

  6. #6
    Chrisrokc's Avatar
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    I would say 44mm would be big enough for a 1300. I wouldn't go bigger than that. On a 1390 or 1500cc setup I think bigger than that could be better.

  7. #7
    Guys i was referring to stock mukunis and the factory numbers..

  8. #8
    (Don't forget the 43 mm TBs that Eric tested had a huge gain in the mid range.)

    Exactly what the 40s do also Philip.... Im anxious to do a back to back and see the difference..

  9. #9
    philip_gpr's Avatar
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    Yes a back to back would be better

    But Eric has been doing this for awhile with both Kawis and Yamahas.

    The modified TBs smoked every other mod he has ever done. I don't think anyone else has reported the TB mod was better than VF IIIS or a Riva head that was further machined to raise compression or ... You get the idea.

    His GPR now smokes his STXR, maybe he can figure out just how much the improvement has been.
    Last edited by philip_gpr; 06-21-2007 at 09:37 PM.

  10. #10
    (Ross have you ever saw the thoat of a 48mm blackjack carb?)

    No ive never owned any Blackjacks Ron, they look awesome.. They must be the exception to the measurement.. A true 48mm..
    Where is your added fuel to match the nitrous fogger?

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