The BST-40 Bible

Discussion in 'Thumpers' started by laramie LC4, May 31, 2008.

  1. UgunsX

    UgunsX n00b

    Joined:
    Feb 23, 2016
    Oddometer:
    1
    Location:
    Latvia
    Here are some pictures of my bikes carb, 640, 2004, 33k km.
    I'm not absolutely shure about this, but I think it might be just slightly worn, eh?? :scratch
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    It seems like none of previous owners ever did any maintenance on the carb. Needle worn almost in half, slide worn badly and guide even has holes worn through.
    Bike is running great, although I have no other lc4 around with mikuni to compare. Runs better than xt660x, but not so good as lc4 with keihin carb.
    When I got this bike, it was all stock except exhaust, which is some aftermarket, don't know exactly what. I did full maintenance, only carb and valves left to do. Air box is stock, with snorkel on top and closed side panel. EPC valve was still there, which I fixed just now. Main jet 152.5, pilot 45. Ordered new parts, they will arrive in two weeks, for now I put the carb back as is, took it for a ride and noticed that now it drops rpm when I twist throttle from iddle just a little bit, some maybe 2mm, twist a bit more and it runs like it should. I guess there is no point in sorting this until all worn parts will be replaced.
    Now, why was I telling all this.. :hmmmmmOh, yes, could anyone tell me dimensions of o-rings? Our dealer has a kit for about 40 EUR, but I only need those 3 rings, I could get them separately but I need to know dimensions.
  2. BeaterBike

    BeaterBike Adventurer

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    Aug 1, 2017
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    Location:
    Vancouver, BC
    @UgunsX if you look back through this thread you'll see the best place for parts is http://www.moto-lab.com/ . The only place I've found with all of the bits for the BST 40, not sure about shipping to Latvia but I'm sure it can be done
  3. Brutalguyracing

    Brutalguyracing Long timer

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    Long Island New York
    welp there is your problem:lol3
  4. motolab

    motolab Long timer

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    Oddometer:
    5,715
    Your DR650 does not originally use a metal shim under the spacer.
    You're welcome.

    Regards,

    Derek
  5. motolab

    motolab Long timer

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    No difference once warmed up points to a rich condition.
    Did you set the TPS after your rebuild and setting the idle mixture & speed?

    Regards,

    Derek
  6. motolab

    motolab Long timer

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    No problem whatsoever shipping to Latvia.

    Regards,

    Derek
  7. motolab

    motolab Long timer

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    Feb 16, 2006
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    You can significantly cut down on slide guide, emulsion tube, slide, and jet needle wear while making significant gains in power by going to a single lift hole late DR650 slide. Gains I measured on the dynamometer were ~1 hp from 5.5K to red line at WOT; across the entire range at 1/4 opening while the mixture essentially stayed the same, with a max gain in torque of 1.4 ft-lbs at 3K rpm; across the board at 1/8 opening, with a max gain in torque of 2.6 ft-lbs at 2K rpm, along with the usable rev range extended from 3.4K to 5K rpm; across the board at 1/16 opening, with a max gain in torque of 1.2 ft-lbs at 2.5K rpm, along with the usable rev range extended from 2.5K to 3K rpm.

    The stock under-diaphragm vent filtration consists of a black plastic cartridge with some screens inside. These screens let a significant amount of dirt through, which accelerates slide guide, slide, emulsion tube and jet needle wear, as well as being ingested by the engine. An oiled foam under diaphragm vent filter solves the problem (see http://advrider.com/index.php?threads/the-bst-40-bible.347184/page-130#post-30339703).
    What is meant by "those 3" (the carburetor uses five o-rings)? In any case, try to get the o-rings in an FKM (aka Viton) material rather than Nitrile (aka Buna-N) so they'll last longer.

    Regards,

    Derek
  8. N3il.H

    N3il.H Adventurer

    Joined:
    Jun 2, 2018
    Oddometer:
    17
    Location:
    Plymouth UK
    Cleaned my carb and noticed the idle screw was set to 1. 75 turns out... I thought that was a bit lean? All jets are stock and still has the snorkel fitted to! I couldn't remove the needle jet or the slide support! They were wedged in tight... So put it all back together and set the idle screw to 2.5 turns out... It didnt want to rev just kept cutting out, set it to 2 turns out and the same, back to 1.75 turns out and its fine! Should i be worried? Im at sea level btw
  9. motolab

    motolab Long timer

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    Adjust the idle mixture via the following procedure:

    Start the engine, let it warm up and ride the bike around until it's hot. Using the idle speed screw, adjust the idle speed to 1450 -1500 rpm (use an aftermarket electronic tachometer if yours does not come with one - you don't have to mount/permanently install it if you don't want to). Set the CO to 3-3.5 %. If gas analysis is not available: coming from the lean side, adjust the fuel screw so that the strongest idle is achieved. You will notice there is a threshold where it becomes rich enough (enough turns out) to run strongest, beyond which no change is noticed. Adjust the idle mixture screw ~1/8-1/4 turn out from this threshold. Adjust the idle speed back to 1450-1500 rpm (as it is likely to have changed). Leave the mixture to the leaner side of these settings if the bike will be seeing altitudes much higher than the one it was set at. Set to the richer side if you would like the engine to idle well earlier during warm up. An extended fuel screw simplifies the process and allows adjustments on the fly if you will be riding at extremes of altitude.

    For diagnostic purposes, report back with the resultant setting.

    What year is the bike and how many miles are there on the carburetor?

    Regards,

    Derek
  10. yahonza

    yahonza Adventurer

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    SE Asia
    TPS was set after the rebuild by the KTM shop. I set the idle and mixture as per your instructions, always ends up at 3 turns out which is higher than the stated factory setting of 2 and a quarter.
  11. motolab

    motolab Long timer

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    Hard to know for sure that it's correct then (nothing against the shop).
    That points to a lean condition which overlaps to idle and is being compensated for with the fuel screw.

    Regards,

    Derek
  12. yahonza

    yahonza Adventurer

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    I don't have any reason to doubt the shop on the TPS.

    So if its a lean condition, what could be the possible cause / solution? Any other potential external factors (not carburettor related)?
  13. motolab

    motolab Long timer

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    I thought you said that the engine doesn't run right.
    Deposits in the pilot jet, incorrectly sized pilot jet, pilot air bleed jet too large, possibly fuel level too low.
    Check TPS setting and perform leakdown test.

    Regards,

    Derek
  14. yahonza

    yahonza Adventurer

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    The only problem is the 'juddering' effect while accelerating in gears above 1st in the 2-3.5k rpm range, so always requires some clutch slippage in town. Engine sounds a little like a chain rattling during this (can only hear if I'm driving past a wall or something so get the reflective sound), then in the higher rpm range, its ok. So for some reason, it always struggles accelerating within this range.

    45 pilot jet. Everything replaced and clean. Float height approx 15.7mm measured with the tool I got from you.

    Will enquire about leakdown testing.
  15. motolab

    motolab Long timer

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    Also check the chain, sprockets, and cush rubbers for wear.

    Regards,

    Derek
  16. yahonza

    yahonza Adventurer

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    Interesting, I replaced the chain and sprockets in 2015 and prob only done 15k km, but have not seen the cush rubbers. Will have a look.
  17. jorijnsmit

    jorijnsmit Adventurer

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    Dec 21, 2013
    Oddometer:
    10
    Big thanks for this guide, well done!
  18. yahonza

    yahonza Adventurer

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    SE Asia
    The cush rubbers do need replacing as there's a slight 'jerkiness' when travelling at a constant slow speed, however that isn't the cause of the power issue. Trying to find somewhere to do a leakdown test.
  19. mxsnow

    mxsnow Adventurer

    Joined:
    Sep 17, 2006
    Oddometer:
    26
    Location:
    Michigan
    I just installed Dereks jetting in my DR,WOW,it woke the bike up big time and it starts hot and cold with just a touch of the button now,I do have a 40 GSXR silencer on it also. Motolab rocks!
  20. recomr

    recomr Adventurer

    Joined:
    Jun 20, 2018
    Oddometer:
    15
    Location:
    Harrisonburg VA
    Have a BST with 3k miles on it. Bike is an 03 625sxc.

    Started dumping fuel out of the overflow (when fuel is turned on) and running rough (lurching, not wanting to start, turning off at low RPMS, and turning off with a quick throttle blip from idle). Had a shop clean the bottom, clean pilot jet, and install a filter but problem still persists. Anybody have ideas? Supposed to be riding the TAT right now... need help asap :confused thanks in advance everyone