BST Needle Help

Discussion in 'Thumpers' started by fire_strom, Oct 5, 2006.

  1. fire_strom

    fire_strom Long timer

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    you guys rock and somner too. i wondering about the slide. how could that much medal be missing with no plasitic loss. anyone know how snug the needle should be in the slide? allowable play? how hard is the needle jet to change? i'll look in the index too.
    scott
    #21
  2. Jan from Finland

    Jan from Finland Been here awhile

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    I can confirm this. My needle broke two months ago at about 70 000 km for
    the same reason. Strange. I can’t say what causes it, but is has worn out
    really smoothly. Grinded by fuel-air mixture, I suppose?
    #22
  3. fire_strom

    fire_strom Long timer

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    when you replaced it did you change out the slide, seat or or any other parts?
    scott
    #23
  4. meat popsicle

    meat popsicle Ignostic

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    I have not seen a how-to on changing the needle jet. Opportunity knocks. :D

    I would guess that if the air fuel mix had enough grit to grind your needle down that much, you would be using a noticable amount of oil... you probably maintain your air filter, and seal the air filter mount so that grit can't get around the filter, so I would look elsewhere for a cause.
    #24
  5. Jan from Finland

    Jan from Finland Been here awhile

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    No, I didn't.
    #25
  6. potatoho

    potatoho Cheese and Rice!

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    Ahh, ok, I see why it wears down now. It's simply because the needle hole is no different than the two holes next to it in the slide. There's a rush of air coarsing through all three holes as the pressures equalize.
    #26
  7. Jan from Finland

    Jan from Finland Been here awhile

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    My bike has never used any measurable amount of oil. Except air filter oil, of course… I did consider grit, but could it be ethanol? They have been mixing it into fuel about a year here.
    #27
  8. meat popsicle

    meat popsicle Ignostic

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    Well Sommer is saying that is a worn needle (pic attached) so I guess y'all are right. 72,000 kilometers or 45,000 miles on the one in their pic. Strange, I thought the needle would wear in the area where it interacts with the needle jet. :scratch

    Attached Files:

    #28
  9. Reformed DucFlogger

    Reformed DucFlogger Inventor Of The Wheel Mk2

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    It's the metal washer(s?) that sits under the plastic donut on the needle.

    The hole in that washer is slightly larger than the dia of the needle (for obvious reasons), and with the washer being steel vs. an alum needle:

    1. The washer wears against the softer needle as vibration and slide throttle movement sees the washer play hoolahoop around needle;
    2. As the needle rubs against the washer as the needle aligns itself (remember that spring?) with center of the hole in the needle jet every time the slide moves downward. As the slide comes down, if the tip of the needle contacts any part of the needle jet, the needle will compress backwards against the spring, rub against the washer(s) going up and very likely as the washer moves on the way back down after finding the center (or a close facsimile thereof) of it's needle orifice.
    3. As the needle bounces up & down x degree thru the washer from vibration, etc.

    The problem would only get worse if the PO added other steel washers of varying diameter (see that 'stepped' wear area on the lower part of the chewed area) to play God with the needle.

    That's my theory and I'm stickin' to it.
    #29
  10. fire_strom

    fire_strom Long timer

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    i would have thought the needle moved with the slide but i guess the spring allows some movment. when i put the needle back into the slide the notch was the same size as the thickness of the bottom of the slide if that makes any sense. so i dont know waht caused it but i i'll be getting a new needle ASAP. i don't use any oil either but the inside of the diafram area of the carb was gritty, i think i'll do the creeper breather filter job soon. on another note, isn't reading BabelFish translations fun in a painful sort of way.:norton
    scott
    #30
  11. potatoho

    potatoho Cheese and Rice!

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    I'd be surprised if it was anything more than a column of air. The smaller washer probably exacerbated the issue by creating a bit less of a seal on the needle hole. The holes are used to equalize pressure in the slide cavity to match the bore, so as it breathes in and out it likely is finding a path via the needle hole as well.
    #31
  12. Reformed DucFlogger

    Reformed DucFlogger Inventor Of The Wheel Mk2

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    It could be the slide plastic, but damn it would have to be some tough stuff.
    #32
  13. laramie LC4

    laramie LC4 flying something...

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    changing the needle jet is quite simple. its just a matter of stripping the carb down, removing it, then repalcing it. when i pull my carb to clean it, i always pull out the needle jet to ensure that everything is clean.

    all you do is, remove the top of carb (needle, washers, slide), then pull the float bowl, remove the float and weight, remove the main jet and spacer, then push down on the inside of the area the MJ just came out of and the needle jet comes out through the intake and into the top of the carb. the only real trick is when you put it back in. there is a "knotch" that only allows it to go in one way. you have to look in and align the slot on the jet with the knotch and bingo, your done.

    give me a few minutes and ill go take some pics, i have my carb out and it will only take a few secs to snap some pics if ya need them.

    laters,

    laramie :beer
    #33
  14. laramie LC4

    laramie LC4 flying something...

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    HERE YA GO.....

    -remove top of carb (2 bolts or screws)

    [​IMG]

    -then remove needle, slide, and washers

    [​IMG]

    -remove float bowl (2 bolts)

    [​IMG]

    -remove float bowl and expose inside of carb

    [​IMG]

    -remove float and weight. DONT LOSE THE WEIGHT!!!!

    [​IMG]

    -remove main jet (flat head screwdriver)

    [​IMG]

    -push down on needle jet

    [​IMG]

    -needle jet should come out threw the intake and towards the top of carb. dont lose the O-ring! if you dont see it, look into the carb, its sits at the top where the plastic piece jsut came out of.

    [​IMG]

    -remove needle jet from carb

    [​IMG]

    -remove needle jet from palstic body

    [​IMG]

    -when reinstalling look for "knotch" on the inside of the carb. you have to line this up with the "slot" on the needle jet. (in this pic i exagerated it and marked it with blue so it was easier to see)

    [​IMG]

    -here is the "slot" on the needle jet. sorry for the crappy pic. my camera sucks at close up, detailed pics... :cry

    [​IMG]

    and that is how you change the needle jet. piece of cake.....

    laters,

    laramie :beer
    #34
  15. fire_strom

    fire_strom Long timer

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    you rock, nice work.
    scott
    #35
  16. meat popsicle

    meat popsicle Ignostic

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    Thank you Laramie.

    Will you be giving this a try fire_strom?
    #36
  17. horse

    horse n00b

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    My needle actually snapped out in the bush and i had to ride back at a constant throttle opening on the choke circuit anyway after a bit of investigation the slide has a step right near the needle hole and the needle wears because the steel washer sits at about a 30 degree angle cocking the needle to the side and constantly stressing against the steel washer when the spring pressure is put on it and the needle is forced into the jet, any carb that i have had apart the needle is parrallel to the slide and free to centralise in the needle jet,if you assemble the bst 40 slide it wants to push the needle one way so i fabricated a little nylon washer so the step is negated, the needle now is central and so far, about a year and quite a few hard dirt miles everything is ok it might not be right but it seems to work and i think the carb is a little more responsive as the friction of the needle to jet is gone (maybe in my head).If this doesnt make sense pull the slide out and look down inside then put some spring pressure on the assembled needle and slide and you will soon work it out,
    Hope this helps,
    Horse.
    #37
  18. Reformed DucFlogger

    Reformed DucFlogger Inventor Of The Wheel Mk2

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    ...great input!

    Next time I have the slide out I'll see about replacing that washer with a plastic or HMM.. (High Molecular somesuch plastic I have laying around - really low friction stuff used in woodworking for jigs, the name of which escapes me).
    #38
  19. fire_strom

    fire_strom Long timer

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    I’ve got an order in for a needle, should be here Wednesday and I’m going to the desert Thursday morning for the weekend so for now, no. I have a rag stuffed in the carb and haven't taken it out to look at the needle jet. I will probably swap it out over the winter, maybe the slide and slide guide too.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:eek:ffice:eek:ffice" /><o:p></o:p>
    scott<o:p></o:p>
    #39
  20. positionsize

    positionsize Been here awhile

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    Great post Laramie! :thumb But for some reason none of the pics in this post will load:(: . The thread is posted the Index so perhaps it is worthy of fixing? :dunno
    #40