The BST-40 Bible

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

  1. motolab

    motolab Long timer

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    That's definitely been cut.
    Looks like they got there today.
    Snorkel removed or race lid installed generally works well with a 157.5 main jet and a 15.7 mm float height. Snorkel removed and race lid installed generally works well with a 162.5 main jet and a 16.7 mm float height. By the looks of it I expect something similar would work for your situation, but it would be hard to know exactly without testing (ideally on a brake dynamometer with EGA). Procedure for selecting the main jet without dyno and gas analysis:

    Install a main jet that is likely to be too large. Test ride the bike at WOT in top gear and note how it runs. Reduce the main jet by one size at a time until the you can just barely tell that the engine doesn't run quite as well as it did. The correct main jet size will be ~2-3 sizes larger than this. Run 100 octane street legal fuel (important that it's street legal, or else it won't tune the same as pump gas) such as Sunoco SS 100 to hopefully prevent detonation problems while on the lean side during tuning.

    Installing an airbox that has not been cut would be the simplest, most direct, and risk free option since the suitable jetting for that is known.
    Sounds good.
    You should improve the exhaust flow as much as possible in order to pull it away from the edge in terms of detonation risk, so if an insert must be installed in order to prevent running afoul (of the Forest Service for example), use the most free-flowing item you can possibly find.

    Regards,

    Derek
  2. MTlost

    MTlost Adventurer

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    Hi all,
    I'm hoping that some of you (with more experience than me) might have some thoughts on my problem.
    New to me, 2005 640 Adventure, 22000 miles, snorkel missing, Supertrapp exhaust can.
    When running in 5th gear between 4 and 5 thousand rpm if I roll on the throttle to pass a car or climb a hill the engine bogs and bucks, if I very slowly open the throttle I can edge the rpms up to 5500.
    It does not seem to be a problem if I go wide open throttle in lower gears, she spins up nicely.

    Any thoughts or ideas on this would be greatly appreciated!

    thank you!
    MTlost
  3. skibum69

    skibum69 slave to gravity

    Joined:
    May 14, 2006
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    9,076
    Location:
    New Melbourne, Newfoundland
    I had a similar problem on a borrowed KLR awhile back and I was certain it was the carb. Turned out the air filter was totally dirty and blocked up which I didn’t discover at all.
  4. motolab

    motolab Long timer

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    At 22K miles, the slide guide and therefore the emulsion tube are guaranteed to be worn out (unless someone has recently replaced them). There are six depressions in the face of the slide guide. Four of these are round. If any of the round ones are no longer visible, there is no need to look any further, the slide guide, and therefore the emulsion tube is worn out. There are two more depressions that look like little parallelograms concentric to the bore adjacent to where the slide bottoms. These depressions are .020" (.51 mm) deep when the slide guide is new. Emulsion tube wear sets in somewhere around .010" (.25 mm) depth, and causes overly rich mixtures in the lower portion of the rpm range (see https://store.moto-lab.com/articles/slide-guide-wear-indicators). How deep are these depressions now (measure with the tail of a caliper - see http://advrider.com/index.php?threa...s-sign-in-please.35110/page-545#post-31501406)?

    Check the slide for vertical grooving on the downstream side, as well as for an enlarged/ovaled needle hole (see https://store.moto-lab.com/articles/slide-wear-indications).

    Has the slide been drilled? You can tell if the insides of the holes are dull rather than satiny, or if the diameter is larger than 2.5 mm. 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.

    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.

    Replace the jet needle if it's worn. This will be evident because missing anodizing will reveal a different color underneath. This is most commonly found right in the area where it passes through the slide (see the needle on the left at https://store.moto-lab.com/articles/jet-needle-wear-example). Also replace the needle if the clip no longer fits it tightly. Install the clip in the 4th groove from the top. The clips are easy to lose, so it's worth getting a spare.

    Make sure that the white plastic spacer fits the needle tightly, that the metal shim is present under the white plastic spacer, and that the shim's hole is not enlarged/ovaled.

    Has the slide spring been clipped? You can tell by whether it has an open coil on one end. If so, it should be replaced, as a clipped spring causes low rpm richness issues.

    Inspect the tip of the float needle with and without magnification to see if it has a witness line where it has been contacting the seat. If so, it should be replaced. Float needles with witness lines cause a higher fuel level, which in turn causes a richer mixture (and at some point overflowing).

    Replace the float seat o-ring if it does not fit snugly or is hardened/deformed/shrunken/cracked/otherwise damaged. A leaky float seat o-ring causes a higher fuel level (and at some point overflowing), which in turn causes a richer mixture.

    If both the float needle and needle seat o-ring are in need of replacement, it's probably best to just get a rebuild kit, as the rest of the rubber parts are likely to also be in poor condition (for example it's virtually guaranteed that the slide guide o-ring and the fuel screw o-ring will be smashed flat).

    Selecting an o-ring that is made from, or a kit that contains FKM (aka "Viton") o-rings will significantly extend the service life.

    Inspect the float hinges for wear (pins should be round instead of lozenge shaped, bores should be round instead of oval).

    Make sure the float is not fuel logged (should weigh 6.1g or fewer when separated from the cage).

    I recommend replacing the pilot jet with a fresh stock sized one (45) as a matter of course. It's difficult to be sure it's clean and they're cheap.

    People often strip the head of the pilot jet. This can be avoided by using a driver with a round shank that closely fits the well and has a hollow ground blade no wider than the head of the jet. There is a purpose-made driver available.

    You will need to set the float height before the float bowl goes back on. Use a float height setting tool (ideally itself set with the tail of a caliper) to accomplish this accurately. The stock setting is 14.7 mm. Instructions with pictures: https://store.moto-lab.com/articles/float-height-setting-tool-instructions-bst40-single-carburetors.

    You can replace the easy to strip and prone to corrosion JIS float bowl, diaphragm cover and throttle cable bracket screws with stainless steel socket heads.

    Replace the inlet fuel filter if the screen is torn. Clean it if it's dirty.

    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).

    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. Report back with the resultant setting.
    Is this the OEM Supertrapp?
    Doe the symptom improve or get worse as the engine warms? What are the jet sizes and jet needle clip position?
    It is normal for symptoms for a given throttle position to be more pronounced under load.

    Regards,

    Derek
  5. MTlost

    MTlost Adventurer

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    Derek,
    Do you offer carb rebuilds if I pull it off the bike and send it to you?
    MTlost
  6. motolab

    motolab Long timer

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    Yes.

    Regards,

    Derek
  7. MTlost

    MTlost Adventurer

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    What is your normal turn around time for carb rebuild?
  8. motolab

    motolab Long timer

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    1-2 business days (plus the shipping time).

    Regards,

    Derek
  9. MTlost

    MTlost Adventurer

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    Aug 29, 2013
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    Derek,
    Would you suspect that the bike is running rich with a worn carb (I'm worried about detonation if its running lean)? I will check my gas mileage and see what I am getting.
    Can you ballpark a cost to rebuild carb?
    Where do I send it to?
    How should it be packaged?
    Anything else I should know?
    MTlost
  10. motolab

    motolab Long timer

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    We should probably be continuing the conversation in private. I'll PM you.

    Regards,

    Derek
  11. MTlost

    MTlost Adventurer

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    Southwest Montana
    ok
  12. alfisto

    alfisto Adventurer

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    Mar 4, 2012
    Oddometer:
    73
    Location:
    Bay Area, CA
    @motolab Back to my DR with the wrong slide installed. I installed the replacement DR-specific single hole/larger cutout slide and reset the mixture screw to 2.5 out, and the bike seems to be idling and coming off-idle more consistently now. I went for a long varied ride to tune and see how it behaved, and mostly was happy with it. Never stumbled off idle or stalled while coasting to a stop, but did stall twice when taking off. Once after a long red light, engine temp at 270F, when I pulled away from the light it choked and died, thank god for E-start being able to get it going again. Another time when I pulled the clutch in while climbing a hill and coasting over the crest, the grade of the hill may have contributed. But overall less stalling and better idling. Still have some fiddling to do with the mix screw, 2.5 out seemed to work best but sounds like the idle mix is still too rich or something else is happening.

    I have to trust I set the float height correctly at 14.7mm with the gauge last time, although from just looking at the horizontal edge on the float relative to the float bowl gasket surface, it seems just a touch below level (as in lower fuel level). I know some people just set the float height with those two level. Other oddities may include the needle setting (currently 3rd position down from top) since the JD jet kit calls for the needle/spacer stackup to only use the OEM white plastic spacer under the retaining clip, no metal shim below the spacer.

    As long as neither of those sound worrying, I'll keep riding and see how it does now with the correct slide. Thanks Derek for the help!
  13. walstibsf

    walstibsf ADVENTURE-HOLIC

    Joined:
    Jul 31, 2008
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    Location:
    Ambrosia Lake, New Mexico/Flagstaff, Arizona
    One thing I've been looking for, but as I remember, one tip for performance was to remove the SLIDE STOP. Item 29 in the following diagram. Seems Creeper or Laramie or somebody had mentioned that in a "un-corking" thread back when....anyway I slept since then and can't find it but plan on doing it anyway to this new-to-me-but-bone-stock 2001 ADV 640.

    [​IMG]

    Thoughts?

    Thanks!
  14. motolab

    motolab Long timer

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    The slide stop was only used in markets with HP-tiered licensing schemes (i.e. not in the US).

    Regards,

    Derek
  15. walstibsf

    walstibsf ADVENTURE-HOLIC

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    HP-tiered licensing schemes? Where HP is limited by a device such as the"slide stop"?
  16. bikemoto

    bikemoto Tyre critic

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    The UK for one, and other parts of Europe. Something like 25 kW / 33 hp for new riders. They are designated as A1 or A2 licences or something, so suitable bikes are often tagged as "A" models. (A UK native might be able to correct me.)

    The slide stop drastically reduces the area of the carb throat. Less air means less fuel means less power.

    Australia and New Zealand also now have a Learner Approved Motorcycle Scheme (LAMS) restricting new riders to 660 cc and 150 kW / tonne. Quite a few bikes are available as LAMS versions, some are able to be de-restricted as the rider progresses. Some specific race-bikes-with-lights are also excluded.

    Enduro bikes in particular have long had these in this part of the world, likely because we get euro-based models. Dealers typically remove the restrictors during the PDI as a matter of course. Sometimes they forget, and someone ends up with a bike well down on power.
  17. BeaterBike

    BeaterBike Adventurer

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    Vancouver, BC
    Thanks for all the help Derek, it's awesome to get such detailed advice on a specific carb, from a specific bike, that's almost a decade and a half old... I'll be installing the new parts on Monday or Tuesday this week and will report back
  18. ya-honza

    ya-honza n00b

    Joined:
    Dec 11, 2007
    Oddometer:
    2
    Question (640 Adv Hi Flo 2005, 66k km). Owned this bike for 3 yrs and have always experienced acceleration 'juddering' at the low revs. Idles fine, no misfire, minimal decel popping, but in my opinion (compared to any other bike i've owned) the acceleration is pretty lame. Have rebuilt the carb to Motolab specs (twice), air filter is clean, plug is new Iridium (changing this made a noticeable difference), TPS is correct, so thinking it has to be something else but what - electronic ignition off? valves (were adjusted last year and was told the cam bearings would need doing next time). Any other suggestions?
  19. motolab

    motolab Long timer

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    What is the airbox configuration? What exhaust is installed? What is the jetting? Does it run differently as the engine warms? How about leakdown test results?

    Regards,

    Derek
  20. ya-honza

    ya-honza n00b

    Joined:
    Dec 11, 2007
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    2
    Hi Derek. First rebuild was all as per standard, but recently i put back the loud exhaust it came with (no name, likely an Asian knock-off. I have the original KTM one also.) and a 157.5 main jet, float height 15.7mm, needle on 4th, removed snorkel. Performs basically the same as the standard set up, just louder really. No difference once warmed up. I keep reading about crisp acceleration on these bikes but have never experienced that on this bike. Have not done a leakdown test.