640 Carb Parts And Bogging

Discussion in 'Thumpers' started by rob748, Jun 8, 2006.

  1. rob748

    rob748 resident alien !!!!

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    BIKE
    05 640 120000 miles.smog off, stock exhaust cored, re jetted, slide drilled, larger main jet, ran great For 6000 miles after that.
    PROBLEM
    So l am having the bogging problem on jumps/hills and l am trying to sort it out ( read all the threads in the index)
    l understand some people have this problem and some don't which l do not under stand, the only thing l can think of is the dirt that has gotten into the top (worn slide) as l never had the problem when new.
    took the carb apart to inpsect, the slide is grooved alot so thinking about replacing it with the diaphram,spring and needle
    dealer quotes
    $38 for the spring :puke1
    $33 for the diaphram
    $9 for the needle
    $38 for the slide
    so $118 and it might fix the problem
    any body have a cheap sorce for these parts ?
    will check the bowl level tonight and t vent the carb and reroute the filters so they do do let all the dirt in like before.
    Reading all the posts l have not seen anyone that has fixed this problem without going to the FCR $$ unless l missed it but at this stage l am on the fence, do l replace the parts $118 or drop the $$ and get the FCR
    l like to ride the bike in all terrain and treat it like a 250 on the trails, the bike is great but this problem is killing me, last weekend l tried to climb a big hill and when l got near the top it got rough and the bike cut out so l had to lay it down rather than watch it tumble down the hill. then l had to slide it down the hill on its side over rocks and other crap (tank looks very pretty at this stage). next time it happens l may not get so lucky so l need to find a fix for this problem
    HELP
    so after finding the parts at cost (thanks pete ) and T venting the carb the problem is fixed.
    First l T vented the vent line that went to the canister at the back and this made a huge difference, also l moved the little filter to under the tank to stop the crap getting inside the top and then new carb spring ,slide and diaphram it is back to what it felt like new and then adjusting it a little it runs great.
    LONG LIVE THE 640 :clap
    #1
  2. Drif10

    Drif10 Accredited Jackass

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    That avatar of yours precludes me helping you at all. :puke2
    #2
  3. ChrisC

    ChrisC Amal sex?

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    My guess is you need a new engine.... :cry

    But seriously folks, the "Mystery of the Bog" has yet to be fully explained. Some contend it's a g-out condition suffered by the vacuum controlled slide, others are in the float level camp. Some have fled to the dark side and installed Beelzebub's carburetor, the much acclaimed Keihin FCR and no longer have an issue.

    As for me, I ride slower than Creeper so luckily enough, I have yet to experience the dreaded "bog".

    CC
    #3
  4. rob748

    rob748 resident alien !!!!

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    its not like lam doing big double jumps, it is happens on small stuff and even climbing hills with little rock jumps on them
    #4
  5. Tholund

    Tholund Adventurer

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    I think you are on the right track...it's probably just time to go thru and do a good cleaning and install a rebuild kit on the carb. If the bike ran fine for a long time...then obviously it's something that degraded...rather than a functional issue. Restoring the conditions should restore the performance.

    Dan
    #5
  6. humdinger

    humdinger bp

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    My '05 did the same thing since new but gearing down to 15 tooth front and a little tuning (valves, air box, exhaust and jetting) the 'bog' is reduced down to approx. 2800 rpm.
    Drop her back a gear or keep revs above this and problem 'bog' gone. Had an XR 600 with a flat slide Mikuni that was the same. Good revs and progressive feeding of the throttle over jumps or rough stuff helps alot. They're not a motorcross carby, not designed that way. I have found weight reduction is helpful also.
    #6
  7. rob748

    rob748 resident alien !!!!

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    when l say bogging l don't mean it is a little slow down below, it bogs enough to send you over the bars and it last for a few+ seconds and even than it is not great for a minute or two.
    done the tooth down ,airbox etc and it was ok for 12000 miles, it did it a couples time s before but nothing like it is now
    how many miles on your 05 ?
    #7
  8. humdinger

    humdinger bp

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    I've only got 3000kms on mine. Whats it like when idling and you quickly blip the throttle? Sounds like fuel delivery at a guess. Have you checked the fuel pump in the tank and all fuel lines,taps etc? When mine bogs, its only very mild but enough to drop the front end over jumps and annoy hell out of me!
    #8
  9. gunnerbuck

    gunnerbuck Island Hopper

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    Rob, perhaps it's an electrical problem, when you hit some rough stuff it activates a short and cuts the spark. Wiggle some wires when it's running and see what happens. My bike had a break inside the keyswitch that caused me all kinds of grief. Good luck.
    #9
  10. rob748

    rob748 resident alien !!!!

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    went out saturday and it was fine until l started jetting airborne then it just got worse, the rider behind me said he could smell fuel
    ordering the carb parts this week and l will see what happens
    #10
  11. holycaveman

    holycaveman Long timer

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    :rofl :rofl :rofl

    Obviously you guys havn't ridden with rob748!:D
    #11
  12. rob748

    rob748 resident alien !!!!

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    see top
    #12
  13. Zerodog

    Zerodog Long timer

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    I think I have solved the bogging problem......at least for the most part. My bike bogged really bad on rocky trails and got worse the steeper the trail got. It would bog on jump faces. Not big jumps just water bars and such. Enough to make me kiss the bars a few times. After a couple of rides I said fuck that and started looking for a fix. So I read all of the stuff here and thought about it for a while. It was my float height. It was about 1/4 of an inch off. How can the float be 1/4 inch off you say????? It is easy with the BST carb. The float seems extra finicky while trying to get it to just touch the needle. If you tip the carb back a little to far you pull the needle out of the seat and trick yourself into thinking you are just touching the little plunger. When I did this the float seam looked parallel to the carb. Make sure the needle is in the seat and.......the float prong is just touching the top of the springloaded plunger but not depressing it. Make sure at this point the float seam is parallel to the carb body. I am 90% sure this is the problem. Maybe they come from the factory hosed up? Maybe it is bad adjustment by someone at some point? After I fixed this my bike doesn't bog on rocky trails at all anymore. And almost never does on the jumps but it still does a little once in a while. I think I am going to drop my float another mm to see if it helps more. But I think I still want an FCR because of the instant throttle. It will act more like my dirtbike then. And hopefully wheelie better on the street.

    This could also mean that you need a new needle and seat . It might not be shutting off. Does you bike smell gassy when just sitting around with the gas on?
    #13
  14. rjf

    rjf SBS's #1 fan in January

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    Mine was bogging as you described. It's much better once I adjusted the float height. Does it slightly every so often, but its much, much better.

    Actually what zero dog said goes for my experience as well.
    #14
  15. meat popsicle

    meat popsicle Ignostic

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    fuel economy is gone to hell, which might be partially due to the puddle that I get from the overflow when the bike is off - sometimes even when the petcock is off :scratch

    Anyways, Sudco says no rebuild kit for the BST40, waiting to hear from CarbParts... but I am thinking I will have to pull the carb and then see what she needs, order - wait :cry

    Perhaps with the rebuild I will sneak in a smaller pilot fuel jet to see if off idle throttle response improves... well, not that it is bad for me but I was always curious if this trick of Dellorto's works.
    #15
  16. bmwktmbill

    bmwktmbill Traveler

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    Call me stupid but my bike doesn't bog('02 640 Adv-BST carb).

    I set the float height by attaching a piece of fuel line to the gas inlet spigot and blow strong mouth breath pressure on the line and raise the float at the same time. It looks a little weird but you can tell very accurately when the flow of air stops and see clearly the attitude of the float. You can have the carb in the operating position(up side down) when you do this.

    This has never failed me and also returns good gas mileage. I have tried all the other ways but always come back to this because it is so precise.
    This works for all float carbs and checks for a good seal on the rubber tip as well.

    Bill in Tomahawk, WI
    #16
  17. Para504

    Para504 Spam, Spam, bacon & Spam!

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    I'm having the same symptoms ('02 Adv). It only happens when I'm riding aggressively - which is good & bad. Bad because its like locking the brakes unexpectedly, usually at the worst time... :doh

    Glad to hear you have it sorted out; I'll be looking at doing the same. Thanks for the updates...
    #17
  18. Kopromayor

    Kopromayor kopromayor

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    Hi from Greece

    Firtstimer here so be gentle please.:ear
    LC4 640 E 00
    Same problem (bogging-stalling) in rough.
    The jump problem existed from the beginning but as many of you mention "the bike was not made for jump"
    But unfortunately or fortunately I race the bike in small rally raids. Around 300km a day in rough roads or offroad. Usually two day race
    The last weekend the race has a 165 km stage then something like regrouping and then 120km special stage.
    There was a jump in the 115km of the special and as I was really enjoying it, I did it. Then the problem appears and as it was uphill I found my shelf pushing.
    Forgot to say there were spectators around.:shog:shog:shog
    Anyway the problem was accessed as the known
    First overall in the first day has made me forget the embarrassing of the stalling.
    Second day: I open the race. In the 20th km the bike hitting very well till then in rough road and fast as you might guess .......stall-cauf-bogg-fuckme.....9 bikes pass (nine minutes back)
    I completed the race like this:
    Seating all the race (230km- my ass still killing me)to avoid jumping around and with the gas as steady as I can like a taxi driver. Any time I tried to push... the same thing.
    Now the bike is in my garage and waits.
    option1.FCR
    option2.clean the carb
    option3.move around the net as I do for 4 days and do nothing:dog


    Thank you
    kostas

    PS if any one wants to see photo from the race please tell.The places are beautifull
    #18
  19. rjf

    rjf SBS's #1 fan in January

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    Post the pics Kostas:clap
    #19
  20. Kopromayor

    Kopromayor kopromayor

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    #20