New musings on BST40 bog

Discussion in 'Thumpers' started by RedWarrior, Aug 1, 2005.

  1. RedWarrior

    RedWarrior dumbfounded dipshit

    Joined:
    Aug 3, 2004
    Oddometer:
    59
    Location:
    Westfield, MA
    I know this problem has been broached before but after having some bogging problem in the bumpies this weekend, I got to thinkin'...

    :tb
    Some have theorized that the bog comes from the slide bouncing up and down obviously bringing the needle with it causing a rich condition. I don't think this is the problem. However, I think it is possible that the needle is jumping a bit off it's seat on the slide when being bounced around. The needle isn't being held in place by anything other than gravity. I was thinking that, if indeed this is the problem, a drop of some adhesive material (I'm thinking a tiny drop of red loctite) to secure the needle to the slide would cure this problem. I'm going to give it a shot and test it out this weekend.

    Good thinking or am I full of crap?

    Red
    #1
  2. creeper

    creeper Still alive...

    Joined:
    Dec 13, 2003
    Oddometer:
    10,718
    Location:
    Puget Sound
    You can never be full of crap if your thinking about why something doesn't work well. You can be wrong, or right, or even "close"... but the fact that you have a theory can only be a good thing.

    Consider this...

    Let’s say for a moment that the vacuum piston "bounces"... and with it, the needle is hopping up and down as well within the vacuum piston.

    Depending on throttle position and air velocity thru the venturi, the theory you pose could have some validity to it.

    A small throttle opening and high RPM will create a substantial low (negative) pressure signal in the venturi. A movement of the needle relative to its "normal" location in the vacuum piston, could cause a rich condition.

    A large throttle opening and low RPM would create the opposite effect in that the pressure signal would be inadequate to pull fuel from the bowl, regardless of the needle position relative to the vacuum piston, in a useable ratio with the volume of air... causing a lean condition.

    To be honest, I don't know how a vacuum piston which weighs well less than an ounce and captured in a relative position by substantial positive and negative pressures, with the constant force of a spring against it as well, could bounce up or down to any significant degree under the forces of gravity and motion in the first place.
    But hey... I could be wrong. :nod

    Back to you RW...
    #2
  3. ChrisC

    ChrisC Amal sex?

    Joined:
    Feb 25, 2003
    Oddometer:
    6,325
    Location:
    Prescott, Arizona USA Earth
    One caveat over and above Creep's essay...

    The needle tends to float in the slide so as to maintain it's centered position in the needle jet. If it's secured by an adhesive it may well bind as it enters the jet. This would result in a slide that hangs up, a condition well known to any Brit car buffs famliar with the Skinner's Union CV carb.... just a thought.
    #3
  4. creeper

    creeper Still alive...

    Joined:
    Dec 13, 2003
    Oddometer:
    10,718
    Location:
    Puget Sound


    Good point Chris.
    If you did "lock" it into position with an adhesive, I'd do it only long enough to test the theory.
    If you could increase downward pressure on the needle ('course, that might be a bad thing too) without increasing pressure on the vacuum piston... that might be an alternative.
    Any method of locking the needle down might create it's own set of problems.
    #4
  5. gaspipe

    gaspipe Wandering Soul

    Joined:
    Jul 12, 2004
    Oddometer:
    11,437
    Location:
    Pickwick Lake, Tennessippi
    Me thinks that unless the needle was perfectly aligned before the glue treatment, it would wear itself, and the needle jet, out with great haste.
    #5
  6. creeper

    creeper Still alive...

    Joined:
    Dec 13, 2003
    Oddometer:
    10,718
    Location:
    Puget Sound

    Yep... wonder which would take the wear first... the hard anodized aluminum needle, or the needle jet? I'm not enough of a DIY metallurgist to know. :tb
    #6
  7. gaspipe

    gaspipe Wandering Soul

    Joined:
    Jul 12, 2004
    Oddometer:
    11,437
    Location:
    Pickwick Lake, Tennessippi
    Me either.... :patch :scratch
    #7
  8. creeper

    creeper Still alive...

    Joined:
    Dec 13, 2003
    Oddometer:
    10,718
    Location:
    Puget Sound
    Brass has some self-lubricating properties... Kinda. Modern hard anodize is some serious tough shit. We'd still have SUs and Strombergs on cars if they had the process back when Chris was... still pretty old. :oscar
    #8
  9. ChrisC

    ChrisC Amal sex?

    Joined:
    Feb 25, 2003
    Oddometer:
    6,325
    Location:
    Prescott, Arizona USA Earth
    Ya know, I just try to add something to the conversation that Mr. Hickson overlooked and there he goes, attacking a senior citizen.

    Has he no shame? :thwak
    #9
  10. creeper

    creeper Still alive...

    Joined:
    Dec 13, 2003
    Oddometer:
    10,718
    Location:
    Puget Sound
    I got shame... or something that passes for it. See?
    #10
  11. ChrisC

    ChrisC Amal sex?

    Joined:
    Feb 25, 2003
    Oddometer:
    6,325
    Location:
    Prescott, Arizona USA Earth
    Who loves ya, Crappy? :wave
    #11
  12. creeper

    creeper Still alive...

    Joined:
    Dec 13, 2003
    Oddometer:
    10,718
    Location:
    Puget Sound
    My daughter, wife and probable future son-in-law do. Well, OK... he could be lying.

    [​IMG]

    Oh yea... my dog kinda likes me, sometimes.

    [​IMG]

    And I know you do Chrissy Poo.

    [​IMG]

    Sorry for the hijack RW... it always goes down the shitter like this eventually.
    #12
  13. potatoho

    potatoho Cheese and Rice!

    Joined:
    Aug 3, 2004
    Oddometer:
    4,709
    Location:
    Sea-level
    The BST40 needle is held under the slide spring tension.

    I've not had a problem with the BST40 stalling after bumps, and thankfully so as I fall down enough as it is.

    I've read through some threads and some claim that venting is finicky, in particular try to keep fuel from clogging them so perhaps loop the bowl vent line upwards. The bowl vent is sticking out on the right side. Mine says "made in Japan" next to it. Looking at it now, I don't believe that bumps could splash fuel that high, though if you fell over to the right side fuel could plug the line.

    The other vent which feeds into the brass elbow has a filter on it. That helps provide atmosphere pressure outside of the slide diaphragm. It also adds a bit of air to the choke circuit. I imagine if that filter were sufficiently clogged, it may affect slide operation.

    There is also a fine filter sticking in the end of the carb's fuel spigot. I suppose this could be clogged or whatnot and slow fuel delivery.

    People have described the BST40 bog as causing them to stall, but with careful throttle control they can recover. Interestingly I didn't see a lot of complaints about hard starting after a stall. Surely, if it stalls due to richness, it will be very difficult to start. I know when I fall over and thus flood the carb bore, it takes me several minutes to get it to fire over. So also consider that this may be a fuel starvation issue.. either due to poor venting, or poor bowl filling rate.

    If the BST40 simply can't keep up with demand during the whoops, you can buy a bowl drain plug with an extra fuel area. I don't know if this would help though. It just has a little extra for the main. I have one which came with a yost kit for my carb.

    This kind of thing:
    [​IMG]

    I saw them for sale somewhere cheap, but now I can't find it. I guess you could see how much Bob will sell one at yostperformance.com.

    I do see advertisements for larger float bowls for other carbs, with claims to eliminate the bogging associated with jumps and whoops. So there may be something to that.
    #13
  14. RedWarrior

    RedWarrior dumbfounded dipshit

    Joined:
    Aug 3, 2004
    Oddometer:
    59
    Location:
    Westfield, MA
    Back from work now and I think I know what will happen if there is needle/jet misalignment. For one reason or another (of which I really have no clue) I had a needle wear out on my Buell in a Keihin carb. It was an aftermarket needle I got in a jet kit from Yost. The needle actually wore such that it looked like a unicorn's horn. Obviously it was rotating around in the jet but I could never figure out why it wore like that. The jet was in perfect conndition. I continued using it with another needle and it hasn't had a problem since. Weird.

    Anyway, it's been a while since I've had the BST apart and seem to remember that Potatoho is right. There is some kind nylon bushing as a base for the spring putting pressure on the slide/needle assy. as a whole.

    Red
    #14
  15. meat popsicle

    meat popsicle Ignostic

    Joined:
    Feb 9, 2004
    Oddometer:
    14,552
    Location:
    Circumlocution Office of Little Dorrit
    bookmark. keep it up ya carbilosophers :getiton
    #15
  16. meat popsicle

    meat popsicle Ignostic

    Joined:
    Feb 9, 2004
    Oddometer:
    14,552
    Location:
    Circumlocution Office of Little Dorrit
    oooh, is that a SeaRay? :evil
    #16
  17. Airhead

    Airhead More human than human

    Joined:
    Nov 22, 2004
    Oddometer:
    2,101
    Location:
    Fear and Loathing in Maine
    You guys are awfully brave to post pictures of the women in your life here...:ilmostro
    #17
  18. MartinDKtm

    MartinDKtm Been here awhile

    Joined:
    Jul 31, 2005
    Oddometer:
    222
    Location:
    Baie-Comeau,Quebec, Canada
    Arctic-Cat sleds of some years had some plug as Potatoho had shown. Built in brass for mikuni carb. I would need to check if I have some at home and see if they fit on the Bst. They where there to prevent water in gas to freeze and plug the main jet and save yourself from an engine seizure from a lean condition. As water would be under the jet level and save yourself. There where also a hose under it so you could drain the carb. If they fit I will post the part number for them.
    #18
  19. Velocibiker

    Velocibiker Adventure Antagonist

    Joined:
    May 28, 2003
    Oddometer:
    1,164
    Location:
    Las Vegas, NV
    I figured I'd just awaken an old one....

    This weekend while attending the RawHyde 'Secret' Adventure Rally, I do believe I've finally experienced the infamous BST bogging. For me, it wasn't due to whoops or jumps (weren't none), it wash washboard.

    Was going about 50-60 over some nicely graded dirt road and rode over some pretty substantial washboard. Prolly 2-3 inches in height, 3-4 inches apart. Felt like the worst cattle guard ever :eek1

    As I hit these patches, it felt like the bike just lost all power. Almost like running out of gas. I think if the patches of washboard had lasted long enough, the bike would have died. Very disconcerting. (psi up at road running pressure and the compression setting a bit high). It was just so weird. I guess I can see a little of what you guys are experiencing. Not good.
    #19
  20. creeper

    creeper Still alive...

    Joined:
    Dec 13, 2003
    Oddometer:
    10,718
    Location:
    Puget Sound
    With a discription like that, it makes you (me) wonder what’s happening to the fuel in the float bowl.
    Is it sloshing up and down so rapidly that a steady fuel supply isn't available, and sucking air the bike leans out... as if running out of gas? A correct float setting might be at issue in this instance.
    Is, under these particular circumstances, the float sticking due to a design flaw unforeseen by the designers of it?
    I have a feeling that for some reason I’ve yet to discover, the delivery of fuel is being temporarily suspended.

    In almost every discription I've read, the victim describes a condition as if running out of gas, yet every armchair carburetion expert tries to link the problem to the vacuum piston... mostly because they don't understand how it works.
    I don't think it's the vacuum piston, diaphragm or needle... never have. Until someone can postulate a logical theory, or in fact prove these parts are the sole cause, I never will.
    (Might be a bias on my part... :lol3)

    There are so many variables. The bike and its condition, functional carb modifications (and “non-functional” carb mods too) terrain, temperature and elevation, gear selection and RPM and the riders use of the throttle, to name a few.

    Velo... did the bike return to a normal running condition immediately after the washboard section, or was there a hesitation or other condition for a while even after the road smoothed out? I ask because the “return to normal dwell time” might be an indicator of the source of the "bog"... vacuum vs. gravity vs. a mechanical condition.

    I have yet to experience this bog. Jumps, washboard, woops… it motors along just fine. Believe it or not, I’d love to experience the condition myself firsthand, just to have a better idea of exactly what is happening.

    Thanks for the excellent discriptive and objective report of the condition Velo. Considering it caught you by surprise rather than it being an ongoing (and irritating) issue, it may be one of the most unbiased discriptions I've heard.
    #20