Setting Float Height on Mikuni BST?

Discussion in 'Thumpers' started by gaspipe, Jul 22, 2005.

  1. gaspipe

    gaspipe Wandering Soul

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    After much searching, this is all I could find. It's somewhat cryptic. Would someone please tell me a bit more? Creeper, Loaded, anyone?

    Am I to assume the upper edge of the float it to be parallel to the gasket surface when inverted?

    #1
  2. creeper

    creeper Still alive...

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    GP... They are refering to the seam on the float where the two halves are joined. That should be parallel to the carb body/bowl gasket surface.
    #2
  3. gaspipe

    gaspipe Wandering Soul

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    Gotcha. Thanks Creeper.

    Now, I'm assuming that this done with the float NOT compressing the spring in the needle......based on the cryptic method in which they hold the carb...?

    :scratch
    #3
  4. creeper

    creeper Still alive...

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    Yep... tab just resting on the plunger, without any compression.

    As I recall :tb... if you do a "wet test" the fuel level should be 2-3mm below the gasket surface when the hose is held beside the center of the bowl.
    Don't hold me to that though. :D
    #4
  5. creeper

    creeper Still alive...

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    Oooops... ment to say "wet running test" not static. :augie
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  6. gaspipe

    gaspipe Wandering Soul

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    Gotcha. Thanks again. I took a pic so other dumbasses like me would know what they are talking about by lookin' at a pic. :nod

    [​IMG]

    I'll do the wet test as soon as the bike is running. And it's close :thumb
    #6
  7. creeper

    creeper Still alive...

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    Nice job on the graphics! :thumb... maybe MeatPop can put in the LC4 List-O-stuff.
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  8. RedWarrior

    RedWarrior dumbfounded dipshit

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    Nice photo. The first time I did mine the float was quite a bit more than "just touching" the plunger. Bike didn't run. :cry Had to pull the carb back out (which I still haven't figured out how to do in a timely or easy fashion) and start all over. Made damn sure I had it right that second time. Been running great ever since.
    #8
  9. gaspipe

    gaspipe Wandering Soul

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    It's a PITA getting the bugger out. I remove the tank, silencer, disconnect the backbone breather, two molex connectors, loosen the airboot clamps, take out the two top subframe bolts to rock it back a couple inches, disconnect the two throttle cables, the choke, and there she blows. Ain't an easy thing to do.

    She was right on, but I have the whole bike apart now anyway and figured I'd check it out and make sure the bowl was clean and stuff. I put a new carb slide 'n guide in, the old one was quite worn.

    The bike will run like crap if the float isn't very darned close. I'm not in love with the 'precision' of this float arrangement, but it works.
    #9
  10. meat popsicle

    meat popsicle Ignostic

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    Huh? what was that part in the middle? :scratch

    bookmarked for da index - thanks GP :thumb
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  11. gaspipe

    gaspipe Wandering Soul

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    :thumb
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  12. potatoho

    potatoho Cheese and Rice!

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    With practice you can check and set the float level with the carb mounted. There's room from the right side. Probably a bit of a pain with the BST because that frame needs to be held.

    I take a zip tie and put some .5mm marks on it and then I temporarily tack it onto the side of the carb, such that I can eyeball the bottom of the zip tie against the float reference point. Then I gently hold up the float until the play is gone and make the changes I want.

    There is usually a recommended range, and in my case there's 4mm to play around with. So far I've been sticking near the lowest fuel level. Just my preference.
    #12
  13. scottcolbath

    scottcolbath Long timer

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    I'm feeling like a retard.

    This procedure does not make sense to me. I'm all apart and ready to rock, only I'm completely confused by this procedure.

    This really doesn't help things.

    Take off the float chamber, press down the float frame and hold

    carburator as shown in the picture. The float 5 mooves downwards.

    Turn carburator slowly until the float moves towards the carburator. In

    this position the edge of the float 6 must be parallel to the sealing

    surface of the carburator 7.

    I can't get an image of this in my head.

    S.C.
    #13
  14. creeper

    creeper Still alive...

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    The idea is that you start out with the float pivoted away from the carb body, then you rotate the carb until the float needle just touches the seat. That's the point from where you measure the float height.

    The needle has a spring loaded plunger... and if you just turn the carb over, the weight of the float can over-power it and give you the impression that the float is sitting deeper into the carb body than it really is.

    Gaspipe's photo gives the impression that the carb is just upside-down... but the carb should actually at about a 35-45 degree angle to measure the float correctly. The "just touching" part is the tip-off.

    C
    #14
  15. scottcolbath

    scottcolbath Long timer

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    That makes sense now. I took a pic of my carb in what I thought was the same position (Carb upside down) and here's what I got.
    [​IMG]

    Thanks Creep.

    S.C.
    #15
  16. creeper

    creeper Still alive...

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    Yep... you'd have to do some serious adjustin' to get it to line up that way. :lol3
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  17. retroone

    retroone Long timer

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    My 2006 640 adventure still boggs after the third or fourth big woop. Should I set the float level lower. I have it set correctly and changed the needle and seat after rechecking the level four times and still boggs. If I have the RPM 7-8k it's ok, but hard to ride it in that range.
    #17
  18. scottcolbath

    scottcolbath Long timer

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    I came to the conclusion that my float may have been coming up a tad short when closing off the flow of fuel. I adjusted and then set up this little Rube Goldberg doohickey I've used on various motorcycles over the years to test whether or not the carb still drips fuel.

    Please pardon the clutter. I need to take a couple of minutes to tidy up.
    [​IMG]

    So far, no drops.
    [​IMG]

    S.C.
    #18
  19. retroone

    retroone Long timer

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    I will answer my own post. I talked to some people that ride the 640's like I do and the BST will not work for my kind of riding. FCR 39 will be on my bike in the near future. Thank you very much!
    #19
  20. gaspipe

    gaspipe Wandering Soul

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    Something to consider: The OE FCR's that come on the Yamahas aren't exactly the same as the ones that come on the KTM's and such. It makes a difference when it comes time to get parts, wherein you have to get soem of the stuff from Yamaha vs. Sudco or CarbPartsWarehouse....

    Are you going for a 39 vs a 41 for the price? The 41 really rocks on 620/640 LC4 with an open airbox and a good exhaust. It gives away *nothing* to the 39 and really howls on the top end.

    If the BST is set up right, you have to be really flying over some very rough ground to make it booger up. I've had four Adventures over the years, and never needed to change needles. I've even run them with cut springs, but found the float height and venting was the issue.

    :scratch
    #20