Mikuni BST40 experts?

Discussion in 'Thumpers' started by Airhead, Jul 11, 2005.

  1. Airhead

    Airhead More human than human

    Joined:
    Nov 22, 2004
    Oddometer:
    2,101
    Location:
    Fear and Loathing in Maine
    The carb on my 2000 LC4 is leaking gas through the drain hose. I checked the drain plug screw, and it is fully closed (I even removed it to see if there were any damaged threads, etc...it seemed to be fine). The gas leak is quite slow with the petcock turned off...maybe one drip every 5 minutes or so. With the petcock turned on, however, the thing pretty much pees gas.

    My question is, is the drain plug screw bad? Or does the drain hose have a secondary circuit that will also drain, say, a bad needle/seat even when the plug is fully closed? I don't want to go replacing the needle if the plug screw is the real culprit.

    One key point...if I run the bike with the petcock closed until it stalls, it does not leak any gas...but if I then open the drain plug screw, gas will empty from the bowl. This leads me to believe that the plug works correctly, and that something else is going on.

    Any opinions? Creeper? Meat?

    BTW, anyone know the part number for the bleeder plug screw? The PO munged up the phillips head flats, probably trying to fix the gas leak by overtightening the drain plug screw. KTM's microfiche doesn't even show the screw as a part...
    #1
  2. creeper

    creeper Still alive...

    Joined:
    Dec 13, 2003
    Oddometer:
    10,718
    Location:
    Puget Sound
    Geeeezzz. Next time I take something (like the carb) off my bike and take it appart... I should really take pictures.
    I can't remember if there is a standpipe for an overflow in the bowl or not. If not, you probably have a damaged screw seat (soft zinc alloy you know) or a porosity, or crack in the drain screw seat area.

    Sorry...not much help. :dunno Maybe someone has a BST on the bench and can pop the bowl off to take a look.
    #2
  3. Mack

    Mack Gone, but never forgotten. RIP, Mack...

    Joined:
    Sep 7, 2002
    Oddometer:
    3,797
    Location:
    Austin, Texas USA Native Oregonian
    Sounds like your float level is set too high. The screw drains the bottom of the bowl, but there is a standpipe that discharges "downstream" of the screw "shut off/bowl drain."
    #3
  4. creeper

    creeper Still alive...

    Joined:
    Dec 13, 2003
    Oddometer:
    10,718
    Location:
    Puget Sound
    I knew that was the brain cell that died recently. :huh

    What the hell would I do without my off-site, auto-run, sub-memory slash hell-of-a-sweet-guy, lovingly known as Mack? More of this :tb no doubt.
    #4
  5. potatoho

    potatoho Cheese and Rice!

    Joined:
    Aug 3, 2004
    Oddometer:
    4,709
    Location:
    Sea-level
    I got one here. I took it apart. The overflow has a ~ 1-1/2" pipe. At the bottom is the drain screw which drains fuel from the bottom.
    #5
  6. Mack

    Mack Gone, but never forgotten. RIP, Mack...

    Joined:
    Sep 7, 2002
    Oddometer:
    3,797
    Location:
    Austin, Texas USA Native Oregonian
    Awww, Creeper :flug







    Sorry musta been the Prednisone talking......

    I'm back to normal now, so its off to the Renazco dungeon for you. :pierce
    #6
  7. creeper

    creeper Still alive...

    Joined:
    Dec 13, 2003
    Oddometer:
    10,718
    Location:
    Puget Sound
    Not for a week or so... :evil
    #7
  8. meat popsicle

    meat popsicle Ignostic

    Joined:
    Feb 9, 2004
    Oddometer:
    14,566
    Location:
    Circumlocution Office of Little Dorrit
    :poser

    where is that big palooka? he takes a pot shot at me then flees! :scratch

    maybe he's making seats for all those poor buggas that have to ride standing up for months! :pot
    #8
  9. Airhead

    Airhead More human than human

    Joined:
    Nov 22, 2004
    Oddometer:
    2,101
    Location:
    Fear and Loathing in Maine
    Thanks guys! I figured there was some kind of overflow drain that would bypass the screw...the symptoms didn't make sense otherwise. I'm replacing the needle/seat and checking the float level as soon as the parts come in...sounds like it will probably fix the problem!
    #9
  10. Airhead

    Airhead More human than human

    Joined:
    Nov 22, 2004
    Oddometer:
    2,101
    Location:
    Fear and Loathing in Maine
    BTW...can you take the bowl off (easily) without taking the carb off the bike? Doesn't seem like you can...

    And, when you check the oil level on bikes with a dipstick instead of a sight glass, do you do it with the dipstick screwed in, or unscrewed?

    Obviously, I didn't get an owner's manual with the bike, and the only one I can find online os for the '03, which has a sight glass.
    #10
  11. meat popsicle

    meat popsicle Ignostic

    Joined:
    Feb 9, 2004
    Oddometer:
    14,566
    Location:
    Circumlocution Office of Little Dorrit
    since having the dipstick unscrewed might lead to inconsistencies from person to person, i am betting its screwed. :D

    I don't remember anyone saying if you can or can't get the bowl off that way. Have a tool that will get the fasteners? might just try it or someone will speak up. There is a few carb removal tricks around here somewhere that should be in the Index Thread... hope someone finds them!
    #11
  12. potatoho

    potatoho Cheese and Rice!

    Joined:
    Aug 3, 2004
    Oddometer:
    4,709
    Location:
    Sea-level
    I removed the bowl with the carb on. I had to use a 1/4" ratchet and bit. The second time I tried to do that though the screws were siezed and I had to pull the carb and fricken cut them off with the dremel. I replaced them with stainless allen heads and spring washer + flat washer and install them only to snug tightness.

    You don't really want to use spring washers directly against the aluminum as it will chew it up. Maybe you can use two spring washers, as I think that's how mine was originally.
    #12
  13. RedWarrior

    RedWarrior dumbfounded dipshit

    Joined:
    Aug 3, 2004
    Oddometer:
    59
    Location:
    Westfield, MA
    You can get the bowl off without too much hassle. I use one of those right angle screwdriver things that have a flat tip on on side and philips on the other. Only problem I had was replacing the idle jet. Thought it was in tight but found out not tight enough when I rolled off the throttle 1/2 mile from home and the bike said "BBBBBbbbbbbbrrrrrrghh... puh!" and quit. Long half mile push. I ended up using a driver from mini screwdriver set and now the jet's in there real good.

    Red
    #13
  14. metaljockey

    metaljockey Dodgy SOB

    Joined:
    Jun 24, 2005
    Oddometer:
    279
    Location:
    Eastern Cape, South Africa
    I know it's an old thread, but it is one that I searched to try and find a cure for my overflowing carb.

    The bike was parked for about 8 months and when I started it, it mysteriously suddenly pissed fuel from the overflow, as long as the motor was running.

    I stripped and checked for sticky floats, checked the float-needle for wear and lastly checked that the float level was set right.

    Re-assembled the lot and still the same problem. Then I took it all out again and this time blew into the fuel line-in and manually opened and closed the needle. The problem was clear to see, the whole needle-seat moved. So the needle seated properly and shut the incoming fuel off, however the seat allowed fuel to bypass it. The seat seats with a little o-ring, which had become hard and allowed the seat to move in it's recess.

    Here's some pictures to show what I mean.

    [​IMG]


    [​IMG]


    [​IMG]

    [​IMG]

    Lastly, I changed those crap screws that keeps the bowl on to stainless allen heads. Just another step to nirvana.

    [​IMG]

    Maybe this helps someone sometime.
    #14
    trautfel likes this.
  15. Brent4ADV

    Brent4ADV Enjoying empty roads...

    Joined:
    Jul 27, 2007
    Oddometer:
    1,006
    Location:
    B'ham, AL
    +1. That's the o-ring that i replaced on mine when I had the leaking fuel problem. Fixed it right away. I think I went to an american sized o-ring as well, which was just a tiny bit larger and it seems to be doing well.
    #15
  16. Blixten

    Blixten n00b

    Joined:
    Jun 3, 2006
    Oddometer:
    8
    I have the same problem. The BST-40 on my 1990 640 adv is leaking gas through the drain hose. I have problem with the fuel pump. And I change to mikuni DF44 instead of Mikuni Rally as it is as original.

    I have switch the o-ring, still leaking. Switch needle and whole needle house, still leaking.

    I could check the float height again, I did not check it after I change needle and needle house.

    So my questions, could it be to much preasure from the fuel pump? The same brand vacum fuel, for 1 carb.

    Any tips?
    #16
  17. dlh62c

    dlh62c Long timer

    Joined:
    Jan 11, 2006
    Oddometer:
    1,603
    My 07 has been doing this for some time, but only when its not running and up on the center stand. Being fundamentally lazy, I've been turning the fuel off at the tank when the bike has to sit for awhile. I've never observed any leaking when the bike is running.

    daryl
    #17
  18. bmwktmbill

    bmwktmbill Traveler

    Joined:
    Jul 16, 2005
    Oddometer:
    5,986
    Location:
    Traveler

    Fill the tank full.

    Connect a hose directly from the petcock to the carb.

    Open the petcock.

    If it leaks without the engine running it is either the needle or the seat or the o-ring...OR...the float is too high.

    Remove the carb by pulling it up through the frame rails.

    Remove the bowl.

    Connect the hose you used for the fuel connection to the carb with the bowl off.

    Blow on the hose and operate the float up and down with your fingers.

    It should seal when the floats are level, then the floats are set correctly.

    If it won't seal at all fix the leak.

    But...
    If it only leaks with the engine running connected normally it must be the pump...but I bet that will not be the case.
    bill
    #18
  19. Blixten

    Blixten n00b

    Joined:
    Jun 3, 2006
    Oddometer:
    8
    Thanks I will test that in the weekend.

    So what is the "normal" preasure from a fuelpump?
    1 bar? or higher?

    I don't think that it should differ much between vacumpumps and I could not find any data about preasure from Mikuni sites.

    Anybody how knows?
    #19
  20. Blixten

    Blixten n00b

    Joined:
    Jun 3, 2006
    Oddometer:
    8
    Yes I have tested to get fuel direkt to the carb, and It's completely dry. So the fuel pump has to high preasure. I will buy the original fuel pump from mikuni instead.
    #20