640 overflow problem

Discussion in 'Thumpers' started by metaljockey, Aug 25, 2007.

  1. metaljockey

    metaljockey Dodgy SOB

    Joined:
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    I have fuel dripping from the carb overflow. It never drips while stationary though. Only when I'm riding. It turns into quite a little stream when I'm riding hard.

    My first question is: is this indicative of too high a float level?

    My second question is: if I want to adjust the float level with the carb in situ, by blowing on the fuel inlet line, should the fuel cutoff still be where the float is parallel to the carb body?
    I know that it should be parallel when touching the needle when the rotation method is used. I'm just not sure whether cut-off and touching is close enough together to give the same result?
    #1
  2. ChrisC

    ChrisC Amal sex?

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    #2
  3. metaljockey

    metaljockey Dodgy SOB

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    I did check out that thread. It didn't answer my question re the difference in shut-off and touching the needle though.
    #3
  4. humdinger

    humdinger bp

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    Hey metaljockey, (spoken in my best South Afrikaan) if you let the rest of us young springboks in on what model carby you have, we will gladly throw our opinions in your direction otherwise you will lose your diplomatic immunity and be forced to wear a rather large shipping container on your head.
    Is that alright with you kaffer?
    #4
  5. metaljockey

    metaljockey Dodgy SOB

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    Sorry, that will be the venerable BST40
    #5
  6. humdinger

    humdinger bp

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    Does it bog when hitting jumps or large woops?
    #6
  7. metaljockey

    metaljockey Dodgy SOB

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    Nope.
    #7
  8. metaljockey

    metaljockey Dodgy SOB

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    Ok, I pulled the carb and the float level was way high. I adjusted it as per manual and now I have a bog off of closed throttle and from idle. And the leaky overflow problem is still there. I fkn hate it when this happens, you try to meddle in shit you dont understand to sort out one problem and suddenly there are new problems lining up to piss you off.

    I'm gonna have to pull it again to try and set it as it was.

    I was surprised to find that the needle appears slightly bent?
    #8
  9. gunnerbuck

    gunnerbuck Island Hopper

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    How many KMs on the bike? I've had to replace my float needle and seat twice after leaking issues. I seem to get 15,000 - 20,000 KM out of a set and then they begin weeping. Once this happens your fuel level in the float bowl will be erratic causing the engine to run rich and making it near impossible to tune properly. Also check to see if the o-ring in the float seat is doing its job.
    #9
  10. metaljockey

    metaljockey Dodgy SOB

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    It's at 11000k's. Thanks, I'll check the needle and seat too.
    #10
  11. humdinger

    humdinger bp

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    I bent the needle on my BST when frigging around with jetting and adjusting things. If you are careful, try straightening it in some 'soft jaws' in a vice. Thats what I did with no probs. The bog of idle may be a number of things but I would start with making sure the float adjustment is right first. I never tinkered with my floats from new and I always had a flat spot on big bumps and jumps, regardless of jetting changes/adjustments.
    This is what I had as far as jetting and other settings were concerned, when I was running the BST 40. This is for sea level to 4000ft elevation and 50 to 99% humidity; my riding area basically:
    Main jet # 157.5
    Pilot jet # 45
    Stock needle; clip set on 4th from top
    Fuel mixture 2 and 1/4 turns out
    Here are some piccys; firstly, a bottom view of the BST and the fuel mixture screw indicated by the screw driver tip. Thanks to my son Zach for holding things while I snapped away :D
    #11
  12. humdinger

    humdinger bp

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    Here's an exploded view of the needle 'arrangement'. Make note of the small white synthetic washer mounted on the needle itself. It is a very tight fit so be careful sliding it down to adjust the clip position as you can bend the needle in doing so:eek1 !
    Also, note the small metal washer that fits between the needle and inside the slide.
    #12
  13. humdinger

    humdinger bp

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    Hereis a piccy of that synthetic washer that is incorrectly mounted too far down the needle...........
    #13
  14. humdinger

    humdinger bp

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    Here is the way it should be............pressed hard up against the clip.
    #14
  15. humdinger

    humdinger bp

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    I am not a big fan of cv carbs so I opted for the Keihin FCR 41mm MX carb, wacked a sexy Akro pipe and muffler, opened up the airbox some more and turned the 640 from a mild mannered cruiser into a fire breathing arm wrencher! :clap
    Hope this helps some.
    #15
  16. meat popsicle

    meat popsicle Ignostic

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    heya metaljock,

    sorry, in a hurry - did ya find the links to setting the float height on the BST40 and float height in general? there are some tips in those threads and links in the index.

    :beer
    #16
  17. metaljockey

    metaljockey Dodgy SOB

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    Ok, bogging problem sorted. As I was loosening the clamps on the rubber thingymebob to the motor I noticed that I never tightened it. No more sucking air, no more bog. Thanks all, leaving on a trip right now. Back in three weeks.

    Wohoo!
    #17