Help re-jetting a bst40 carb for a xr600r

Discussion in 'Thumpers' started by xrinbits, May 13, 2013.

  1. xrinbits

    xrinbits n00b

    Joined:
    Apr 3, 2011
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    7
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    Dorset UK but trying to escape
    Has anyone on here fitted a bst40 Mikuni (mine came from a Ducati 640) to a xr600r only I need some help.

    I fitted the Mikuni because with my standard Keihin carb the bike takes too much kicking to start it, I've stripped and renewed jets, gaskets, 'o' rings etc. but with no success.

    The good news is it starts second kick if not firs kick every time, the problem is it seems to run poorly above 1/3 throttle, its like its starved of fuel just like when you know you have to switch to reserve on the fuel tap.

    I should also say that I've been in conversation with motolab on this forum and shown him photos of the internals of the carb and it checks out ok.

    The original owner swapped for fcr carb and the Mikuni was working fine on his bike prior to removal .

    The fitted jet sizes are :-

    Idle 45
    Main 152.5

    Please feel free to comment all help gratefully accepted.
    #1
  2. sp600

    sp600 Adventurer

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    #2
  3. laramie LC4

    laramie LC4 crash test dummy!

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    i would say you need to look at the needle.

    my $.02,

    laramie :beer
    #3
  4. xrinbits

    xrinbits n00b

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    Ok here's an update:-

    New 165 main jet fitted but still does the stumbling/misfire at about 1/3 to 3/4 throttle, occasionally it seems poor acceleration right through the rev range.
    But when working well it pulls and accelerates like crazy :D and its never been better.

    I've sent for a range of pilot jets as the one that's fitted is standard so maybe that will help as I think I read somewhere that the pilot has an effect up to about 3/4 throttle (someone please comment if that's wrong)

    If anyone has any suggestions please chip in.
    #4
  5. xrinbits

    xrinbits n00b

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    Hi Laramie,
    What did you mean by look at the needle do you think its a position issue.
    #5
  6. xrinbits

    xrinbits n00b

    Joined:
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    Finally sorted the carb problem, I fitted a bigger main jet and pilot jet but the main difference came when I dropped the needle by 2 notches.
    Not quite the same amount of arm wrenching acceleration but at least its very usable and starts with a maximum of three kicks.

    Thanks to all who helped .

    Andy:1drink
    #6