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The BST-40 Bible

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

  1. motolab

    motolab Long timer

    Joined:
    Feb 16, 2006
    Oddometer:
    5,907
    Did you try driving it out with a mallet (after removing the spacer of course)? It may help to ultrasonically clean the body first, and to wick in some penetrating oil around the perimeter.

    Regards,

    Derek
  2. Crab

    Crab n00b

    Joined:
    Feb 28, 2018
    Oddometer:
    1
    Location:
    Ohio
    What Derek said...I put a little kerosene around the edges of the slide support and left it overnight. Got it out the next morning.
  3. Zapp22

    Zapp22 ZAPP - Tejas

    Joined:
    Oct 20, 2004
    Oddometer:
    13,857
    Location:
    Tejas Hill Country
    THANK YOU for posting this original.
    This now being Fall [officially!] 2018, I want to keep my '97 "mostly stock" - the only mod I ever made was putting in proper stainless inline filter, and installing a A/F mix thumbscrew years ago. She has run excellently until about a year ago - the carb float or the O-ring are giving trouble and it seems like the "slow jet" is compromised so its time for a rework. WHAT KIT, or what group of parts is recommended as of this date to fix the pilot, probably going up one notch there but leaving the main stock - no airbox mods. ??? I need to place and order
  4. motolab

    motolab Long timer

    Joined:
    Feb 16, 2006
    Oddometer:
    5,907
    How many miles are there on the carburetor?

    Regards,

    Derek
  5. Zapp22

    Zapp22 ZAPP - Tejas

    Joined:
    Oct 20, 2004
    Oddometer:
    13,857
    Location:
    Tejas Hill Country
    11000 miles. i know.... its shameful, but I have a vstrom also... and had a 02 DR, and another 'strom, and a BRP
  6. motolab

    motolab Long timer

    Joined:
    Feb 16, 2006
    Oddometer:
    5,907
    I should have asked before, has the slide been drilled or the spring been clipped? What jet needle is installed? Is the stock under diaphragm vent filtration still in place? Is the bike ridden predominantly in the dirt or on the street?

    Regards,

    Derek
  7. Diddly

    Diddly Bored of Directors

    Joined:
    Dec 17, 2015
    Oddometer:
    347
    Location:
    Indiana
    Big endorsement for Derek at Motolab. I called to get refresh parts for my BST. He helped me select the parts I needed and skip the ones I wouldn’t need. Shipped cheap and still arrived day-plus. Every time I get Motolab deliveries, each component arrives in individually labeled baggies. A nice touch for those of us who need help keeping parts straight.

    Posting more on other exhaust help over on the ‘640 owners sign in’ thread.

    Thanks Derek.
    bikemoto, Whodatschrome and dmsantam like this.
  8. motolab

    motolab Long timer

    Joined:
    Feb 16, 2006
    Oddometer:
    5,907
    Sir, you are very welcome, and thank you for the kind words!

    Regards,

    Derek
  9. MTlost

    MTlost Adventurer

    Joined:
    Aug 29, 2013
    Oddometer:
    79
    Location:
    Southwest Montana
    Diddly is right! Derek is great!!

    MTlost
  10. motolab

    motolab Long timer

    Joined:
    Feb 16, 2006
    Oddometer:
    5,907
    Thank you. Thank you very much.

    Regards,

    Derek
  11. Mugenski

    Mugenski n00b

    Joined:
    Feb 7, 2010
    Oddometer:
    4
    Hey Derek and fellow ADV riders.

    Coming looking for advice regarding my '04 BST40 on my DR650 with a DJ kit. Here is how my bike is setup:

    36k km, Stock header with GSXR x040fx, snorkel removed, side airbox cover removed, twinair filter. Elevation @ 260m. Aftermarket breather filter seems to have kept slide wear to a minimum.

    I followed MXRobs pointers to get the DJ needle to sit properly. 2nd notch using filed down white spacer from stock needle so it's sitting properly at 4th notch equivalent height. Paired with 160DJ main jet (150 mikuni). I have a larger DJ main at 170 (mikuni 160) but that seems a little much from what I've read.

    I started with the main jet and needle change, took it out for a spin with a noticeable improvement from 1/4-WOT. I played with the fuel screw between 1.5 - 2.25 turns to see if I could improve the low end of the throttle (0-1/8) or so. Perhaps felt a tad peppier right off the line around the 2t mark - also had less decel pop.

    Next I drilled out the slide and opened both holes to 2.7mm (7/64) and clipped 3 turns from the spring. Bike felt stronger off the line and definitely revved easier but I am still looking for a little more on the low end of the throttle just as you get off idle. People talk of a well tuned BST pulling the wheel up in first and second and I seem to be close. Any ideas? Pilot jet is stock I believe at 42.5 and I haven't tried taking the fuel mix screw much past 2.25 turns as I assume that will just make my idle overly rich.

    Any input is appreciated!
  12. motolab

    motolab Long timer

    Joined:
    Feb 16, 2006
    Oddometer:
    5,907
    The Dynojet needle's drawbacks are:

    1) It was designed for the '80-'83 Yamaha XS & XJ1100 and/or the '80-'85 Suzuki GS750E & ES.

    2) It has a shape that is likely to produce significant variances in mixture strength through the rpm range, specifically leaner toward upper portion of the rpm range than toward the lower.

    3) It's made out of stainless steel which means more emulsion tube wear than with an aluminum needle.

    4) It doesn't use the white plastic spacer under the clip, which means the needle is not preloaded in the downstream direction, and will therefore be more prone to orbiting/otherwise gyrating inside the emulsion tube. It also means that the needle will not be able to pivot out of the way as the slide guide wears. Both of these factors equate more emulsion tube wear. Note: studying Dynojet needle applications reveals that they keep OEM spacers in place unless, as in this case, the needle is adapted from a use on a different carburetor.
    In light of the multitude of reports of audible detonation on stock (and other) DR650s, and that a DR650 I tested on the dynamometer with a BST40 carburetor, cut airbox, an FMF slip-on with the insert removed and jetted correctly had NOx values that I would consider to be out of bounds in terms of detonation risk, I wouldn't consider cutting a DR650's airbox without a significant improvement in exhaust flow. Removing the lid is likely to increase cylinder filling above and beyond what airbox cutting does.
    What are the depths of the depressions in the slide guide in the bottom corners adjacent to the bore?

    [​IMG]

    Zero a caliper here (sorry for the blurry image):

    [​IMG]

    Find and measure the lowest spot in the immediate surrounding area. Then repeat for the other side.
    Set the fuel screw to 2-1/2 turns out to start. 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 1500 rpm (use a digital tachometer). Adjust the fuel screw so the CO is ~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 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 helps to simplify the process.

    For diagnostic purposes, report back with the number of turns the screw ends up at. Some amount of decel popping is normal, and should not be attempted to be corrected for with an idle mixture setting that is other than correct for idle. Off idle issues should also not be attempted to be remedied on the overlap via the fuel screw, but rather by addressing the causes.
    Slide guide wear is pretty much proportional to lift hole area, so drilling the slide will dramatically increase slide guide, emulsion tube, and slide wear. Dynamometer testing I have performed on a late model DR650 also showed that it causes a loss in hp at 1/4 opening and richens the mixture at 1/8 opening in not easily correctable manner.

    Keep in mind that between every intake pulse, the slide spring and gravity try to push the slide back down. In order for the slide to be allowed to be pushed back down, the vacuum above diaphragm has to bleed off through the slide lift holes. The larger the slide lift hole area, the faster that vacuum can bleed off, and therefore the greater the distance the slide can be pushed down before the next pulse. At the next pulse, the slide has to be pulled back up by the distance it fell (more if there is acceleration, less if there is deceleration). Conversely, the smaller the lift holes, the slower the vacuum can bleed off, and therefore the smaller the distance the slide can be pushed down before the next pulse, and therefore the smaller the distance the slide has to be pulled back up at the next pulse.

    Inmates have also complained about surging in cross winds after having drilled the slide.

    Cutting the spring reduces the preload while increasing the spring rate. The result is that the slide rides higher for a given intersection of throttle position and rpm early in the travel, and because the amount of air allowed in the engine in this area is largely controlled by the butterfly, the result is typically a richened mixture. Later in the travel (i.e. at larger openings and higher rpm) the increased spring rate is likely to lower the slide for a given intersection of throttle position and rpm.
    The off-idle mixture is likely to be too rich since several of the changes you made richen the mixture in that area. This will be compounded by the fact that at 36K km it would be very unusual for the emulsion tube not to be worn.

    Also, float height is a very important component of getting a BST40-equipped DR650 to run properly. Testing on the dynamometer has shown that it must be set to exactly 14.7 mm, or else the mixture will be off across the span of several openings. DR650s are inordinately sensitive to changes to float height, with 1mm being worth ~2.4% CO, which is the equivalent of approximately two jet sizes. As far as I'm concerned, the only way to set it accurately enough is using a float height setting tool, itself set to the target via the tail of a caliper (see http://advrider.com/index.php?threads/the-bst-40-bible.347184/page-140#post-31898291).

    Regards,

    Derek