BST Needle Help

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

  1. fire_strom

    fire_strom Long timer

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    I have the needle out of the carb and there is a large notch out of it. any idea what might have caused this?

    [​IMG]

    i am a carb noobie and i was trying to get rid of the backfire on deceleration i am experianceing. i have had the needle out before but i don't remember if it had the this notched section. i also lost track of the order of things, i think i have it lined up right in the picture but the manual doesn't show the little washer on the right, any idea where that goes?
    thanks
    scott
    #1
  2. fire_strom

    fire_strom Long timer

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    more questions, the circlip was in the center (stock) notch, i was going to raise the needle one notch per flanny's guide does this richen the mix through out the throttle range or only in a particular section.
    thanks again
    scott
    #2
  3. Zerodog

    Zerodog Long timer

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    Damn! That thing is worn out. I don't think the little washer on the right is supposed to be there. It could have been vibrating around and caused the damage? You will need to get a new needle for sure.
    #3
  4. laramie LC4

    laramie LC4 flying something...

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    raising the clip will richen the entire circuit but it will mostly affect the 1/4-3/4 throttle range, which is where the needles does it's work.

    unfortuntaly jetting isnt as simple as "make a change here, and it only affect here." all of the circuits work together and are affected by the other, but at the same time each circuit needs to be correct for its specific range.

    as for your needle, i have no idea what could have happened there ? :eek1 ive never seen anything like that before. it also appears that you have an additional washer. here is a schematic of the needle and how it should be arranged....

    [​IMG]

    you should have the clip, then the plastic washer, then the metal.

    why you have more than that, i dont know. it could be that at some point someone halved the washers to allow for more "fine tunning" but if its always been your bike, im not sure what to say....

    as for the backfire, this is one of those things that you can chase forever and never get rid of. i have my carb jetted and tuned about as well as a guy can have and i still get a "pop" when i back off the throttle quickly. i personally believe it has more to do with the CV carb than with poor jetting.

    good luck and let us know what you find out.

    laters,

    laramie :beer
    #4
  5. SPEEDSMITH

    SPEEDSMITH Sh!t Disturber

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    I'm not sure what caused it, but, there is something wrong for sure.
    I've worked on tons of carbs, and have never seen a needle that worn.
    Is that the stock needle, or a Dynojet one?
    I've seen lots of worn DJ needles, but nothing like that!
    The small washer is used to make 1/2 notch adjustments.
    #5
  6. meat popsicle

    meat popsicle Ignostic

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    The Jet Needle is not designed to, ah, nevermind.

    Make sure your exhaust system is free of leaks, as I hear those can cause backfiring upon deceleration. Can anyone give us a technical review of the phenomenon of backfiring upon deceleration? :ear
    #6
  7. herobikes

    herobikes HE AINT HEAVY HES MY LC4

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    Go to mikuni.com , click on carburators , then tuneing guide , pick your topic from there , good info .
    #7
  8. laramie LC4

    laramie LC4 flying something...

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    here is what the manufacuter says;

    <TABLE cellSpacing=0 cellPadding=0 width=570 border=0><TBODY><TR><TD vAlign=top colSpan=33 height=125><BASEFONT size=2><TABLE cellSpacing=0 cellPadding=0 width=565 border=0><TBODY><TR><TD height=58><BASEFONT size=2>[​IMG]</BASEFONT></TD></TR><TR><TD height=26><BASEFONT size=2><TABLE cellSpacing=0 cellPadding=0 width="99%" border=0><TBODY><TR><TD vAlign=top align=middle width=66><BASEFONT size=2>
    [​IMG]
    </BASEFONT></TD><TD vAlign=top align=middle width="20%"><BASEFONT size=2>
    [​IMG]
    </BASEFONT></TD><TD vAlign=top align=middle width="3%"><BASEFONT size=2>[​IMG]</BASEFONT></TD><TD vAlign=top align=middle width="10%"><BASEFONT size=2>[​IMG]</BASEFONT></TD><TD vAlign=top align=middle width="3%"><BASEFONT size=2>[​IMG]</BASEFONT></TD><TD vAlign=top align=middle width="23%"><BASEFONT size=2>[​IMG]</BASEFONT></TD><TD vAlign=top align=middle width="3%"><BASEFONT size=2>[​IMG]</BASEFONT></TD><TD vAlign=top align=middle width="10%"><BASEFONT size=2>[​IMG]</BASEFONT></TD><TD vAlign=top align=middle width="3%"><BASEFONT size=2>[​IMG]</BASEFONT></TD><TD vAlign=top align=middle width="10%"><BASEFONT size=2>[​IMG]</BASEFONT></TD><TD vAlign=top align=middle width="3%"><BASEFONT size=2>[​IMG]</BASEFONT></TD></TR></TBODY></TABLE></TD></TR><TR><TD height=26><BASEFONT size=2><TABLE cellSpacing=0 cellPadding=0 border=0><TBODY><TR><TD><BASEFONT size=2>[​IMG]</BASEFONT></TD><TD><BASEFONT size=2>[​IMG]</BASEFONT></TD><TD><BASEFONT size=2>[​IMG]</BASEFONT></TD><TD><BASEFONT size=2>[​IMG]</BASEFONT></TD><TD><BASEFONT size=2>[​IMG]</BASEFONT></TD><TD><BASEFONT size=2>[​IMG]</BASEFONT></TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE></TD></TR><TR><TD vAlign=top colSpan=33 height=455><BASEFONT size=2>[SIZE=-1][​IMG]
    [/SIZE]<TABLE cellSpacing=0 cellPadding=0 width=565 border=0><TBODY><TR><TD width="100%"><BASEFONT size=2>[​IMG]

    [SIZE=+1]4: Backfires in Exhaust
    [/SIZE]

    Note:

    It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

    The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

    Why This (normally) Happens:

    <TABLE cellSpacing=0 cellPadding=0 width="100%" border=0><TBODY><TR><TD vAlign=top width="3%"><BASEFONT size=2>1) </BASEFONT></TD><TD vAlign=top width="97%"><BASEFONT size=2>When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.</BASEFONT></TD></TR><TR><TD vAlign=top width="3%"><BASEFONT size=2>2)</BASEFONT></TD><TD vAlign=top width="97%"><BASEFONT size=2>The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.</BASEFONT></TD></TR><TR><TD vAlign=top width="3%"><BASEFONT size=2>3)</BASEFONT></TD><TD vAlign=top width="97%"><BASEFONT size=2>Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.</BASEFONT></TD></TR><TR><TD vAlign=top width="3%"><BASEFONT size=2>4)</BASEFONT></TD><TD vAlign=top width="97%"><BASEFONT size=2>When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.</BASEFONT></TD></TR><TR><TD vAlign=top width="3%"><BASEFONT size=2>5)</BASEFONT></TD><TD vAlign=top width="97%"><BASEFONT size=2>Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.</BASEFONT></TD></TR></TBODY></TABLE>

    [SIZE=-1]Other possible causes:

    Air Leaks:
    [/SIZE]
    [SIZE=-1]
    Any source of fresh air into the exhaust system can create or worsen the conditions that bring about exhaust backfiring. The most common entry point is the junction of the header pipes and mufflers. Even a small air leak can dramatically increase the intensity or likelihood of exhaust system backfiring.

    A high temperature silicone sealant, as can be found in many auto parts stores, may be used to seal the pipe/muffler junction. [/SIZE]

    [SIZE=-1]Lean Carburetion:
    [/SIZE]
    [SIZE=-1]
    While exhaust system popping may be considered normal, it is certainly made worse by an overly lean idle circuit.

    Be sure that your carburetor's pilot jet is the correct size and that the idle air mixture screw is correctly adjusted before looking for other causes of popping. The procedure for adjusting the pilot circuit is covered in the Tuning Manual.[/SIZE]

    [SIZE=-1]Ignition:
    [/SIZE]
    [SIZE=-1]
    If exhaust system popping is very loud, irregular and accompanied by loss of power, then you should suspect that the ignition system is not performing as it should. If, for some reason, the ignition sometimes fires at the wrong time, then exhaust popping can become very energetic (loud). Look for failing high tension leads (plug wires), failing ignition coil(s) and especially switches or connectors as possible causes.[/SIZE]

    </TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE>
    #8
  9. firerigger

    firerigger Trauma waiting to happen

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    Holy shit! Quit taking your bike apart already, or we'll never make it to Utah! But seriously... :rofl
    #9
  10. potatoho

    potatoho Cheese and Rice!

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    Wow that's odd. I just made a mark on my needle and the wear on yours is actually above where the needle is exposed. I've got no wear at all in that location, however I do have some wear at about halfway down the needle.
    #10
  11. Greg Podlaski

    Greg Podlaski fat ass

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    I would guess the damage was caused by the additional washer having such a large hole and vibrating against the needle
    #11
  12. laramie LC4

    laramie LC4 flying something...

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    you'd would think that it would be all the way around the needle and not just on one side if that was the case. to me it looks like someone took a file to it for some unknown reason ??? :huh

    laramie :beer
    #12
  13. Greg Podlaski

    Greg Podlaski fat ass

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    Maybe to lighten it up to get the slide to move faster,they might be on to something:rofl
    #13
  14. meat popsicle

    meat popsicle Ignostic

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    Thanks hero and laramie; good info in that tuning guide at Mikuni.com
    #14
  15. fire_strom

    fire_strom Long timer

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    that was really my first thought. then the sadness of BS rapidly descended. from what i can tell now it seems that the notch or chunk that is out of the stock needle fits just right in the slide. like it was getting pushed against the slide there and vibrating. when i put it together now (with the mystery mini washer right below the cir-clip) it sits cockeyed, like it doesn't want to go straight into the seat. for now i will just order a new needle and leave the extra mystery washer out. the that mini washer by the way has the exact same size hole as the plastic spacer and the approved washer. it looks bigger due to the optical illusion. that's part of whats weird, it fits perfectly on the needle like it wants to be there but not really as a half spacer, the OD is too small:norton.

    WTF

    maybe this crazy quandary will lure creeper out of this sabbatical with some knowledge else wise secret :pope knowledge.

    man I'd hate to have to punt and go FCR.:lol3

    don't worry shane UT is still a go even if i have to ride with a propane bottle between my legs and a hose shoved into the intake. and you though i had throttle control issues before:rofl

    scott
    #15
  16. fire_strom

    fire_strom Long timer

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    and thanks for the mikuni carb link, i was on that site today but was diagram obsessed so i didn't even see the tuning guide.
    scott
    #16
  17. potatoho

    potatoho Cheese and Rice!

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    You can't blame the guy for improvising LOL. Most of my shims are cut out of bubble pack plastic I had laying around.
    #17
  18. taffyscouser

    taffyscouser Adventurer

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    Look on the Somers KTM site as there is a needle just like that in the LC4 section. Do not know what they are saying about it as its in German. Are you sure its a standard part. They have a little box diagram of a needele like that in one of their tips sections on the LC4 engines. Check the catalogue section as well, as they also have one on there.
    #18
  19. laramie LC4

    laramie LC4 flying something...

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    here is what they have on th Sommer website;

    damit, its not letting me paste the pic....

    but this is what it says;

    DÜSENNADEL



    für Mikuni BST 40

    sehr wichtiges Austauschteil nach 20-30
    tausend KM,
    (5556) € 5,85
    Vergleich Laufleistung 72.000 km und neues Exemplar

    roughly translated, this means;

    Nozzles needle for Mikuni BST 40 very important exchange part after 20-30 thousand KM, (5556) € 5.85
    comparisons course achievement 72,000 km and new copy

    so it looks like its due to wear and tear, go figure. i was wrong....???:D

    if you want to see the pic, go to


    http://www.ktm-sommer.de/SP_PDF_06/04_VERG_023_035.pdf


    laters,

    laramie :beer







    #19
  20. gunnerbuck

    gunnerbuck Island Hopper

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    While that needle was wearing what did it do to the receiver hole in the slide? You may have to look at replacing that as well. I replaced the needle jet and the jet needle due to wear on my bike recently, it showed wear , but nothing like yours, it was way down the needle.
    #20