The BST-40 Bible

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

  1. motolab

    motolab Long timer

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    This would be more pronounced when the engine is hot.

    Regards,

    Derek
  2. 2 SPOT

    2 SPOT wannabe

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    not always.
  3. motolab

    motolab Long timer

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    Name such an instance and the reason for it.

    Regards,

    Derek
  4. 2 SPOT

    2 SPOT wannabe

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    i cant, i pulled that suggestion right out of my ass with absolutely no background or experience to come up with it,,,,,

    so sorry, wont happen again.
  5. Tonicu

    Tonicu Been here awhile

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    I posted this on the incredibly good LC4 thread, but maybe this one is more appropiate. Please excuse me for the copy&paste. Pictures here :
    http://www.advrider.com/forums/showthread.php?t=86958&page=312

    I found some time to pull the carb after the bike completely stopped and refused to start again when at single track steepy sections. As usual, it run fine on road but not so fine offroad, continously stalling.
    The carb was full of gas, flooded. Apart from the weird colour of the needle all the rest looked fine. The needle was in the 4th slot from the top ... before I lost its circlip.
    A few of you suggested it could be running lean and causing overheating, but taking a look inside the carb it doesn't seem to be the case.
    I was hoping to take a look at the jets but haven't yet succeeded in unbolting those butter screws at the bottom of the carb.
    This carb is a second hand one with only 12k kms, plus another 5k I've done, since the older one needed a new slide&guide, membrane and needle at 50k kms.
  6. meat popsicle

    meat popsicle Ignostic

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    Its real easy to misconstrue a person's tone or intent in forum posts. Thanks to all for throwing their ideas on the table.

    2 SPOT: Valves were adjusted by Scuderia West less than a thousand miles ago (I didn't have time to do it myself, but they are a good shop). If they've slipped this quickly I'd be real pissed.

    bill, I'm guess if the new exhaust can was a dud it would effect how it runs when hot too. I'm going to chase down these other possibilities first (air leaks, old jets, fine tuning). Although I would just pull the exhaust can and test cold start before running water through it.

    Now I did have a paper towel blocking the engine-side carb boot; I'm pretty sure I pulled it before shoving the carb back on... but wouldn't a blockage enrich the air-fuel mixture like a true choke and interfere with warm running?
  7. 2 SPOT

    2 SPOT wannabe

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    we'll assume the valves are in spec, next i go to a low float height.

    my suggestions are based on the bike running fine before you didnt really do anything but a mild carb rebuild including moving the float height, you didnt say how the float height was set but i am thinking you found it high and lowered it?
  8. motolab

    motolab Long timer

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    Low float height equals high fuel level.
    My advice to Meat was and is to set the float height between 14.7 (standard) and 16.7mm. The reason for suggesting more than 14.7mm is because the mixture on BST40-equipped 620-660 LC4s tends to be richer toward the bottom (in terms of rpm), which will be exacerbated by opening the airbox and installing the correct main jet to go with that (not recommended on pre-highflow models unless significant measures have been taken to prevent detonation), as well as by emulsion tube wear (over time). That said, a strange float height adjustment alone will not cause the idle mixture to be so far off that adjusting the fuel screw can't correct it. If anything, a low float height will cause the engine to start more easily from cold.

    Regards,

    Derek
  9. motolab

    motolab Long timer

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    I think this one is more appropriate.
    Make sure that the float seat is clean, free of debris and undamaged. Replace the float seat o-ring if it is cracked/shrunken/deformed/hardened or otherwise damaged. Replace the float needle if there is a witness line evident under magnification where the tip has been contacting the seat. Replace the float assembly if one or both of the floats are fuel logged (this happens rarely). Using a float height setting tool, set the float height to 14.7-16.7 mm.
    Does it run better when the engine is cold, or when it is hot?
    Use a hand impact driver with the appropriately sized JIS bit. Reassemble with socket head cap screws.

    Regards,

    Derek
  10. Mo-Tarded

    Mo-Tarded Been here awhile

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    I was thinking the same thing... If it has sat for any length of time perhaps it has ethanol poisoning.
  11. Tonicu

    Tonicu Been here awhile

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    Thanks for your tips, Derek.

    As for the question of whether it runs better hot or cold I'd say it runs better when cold, despite it doesn't start as it normally did, just pulling the choke lever for a few seconds and then idling smoothly.
    The new starting procedure is pulling the choke and hit the button, but the engine doesn't start until I put the choke lever into rest position, then starts and idles more or less fine.
    When hot it starts at the first touch and runs fine, although sometimes it seems to run better, even seeming to be making more power.
    Trouble begins when at single track sections, those steepy, bumpy and rocky ones.
    Only in those sections the engine seems to starve from fuel, eventually stalling, but surprinsingly restarting as if nothing had happened after a few seconds.

    I suffered the same problem with the old BST, thought all the problems were gone with the "new" one but here they come again.

    Another hint might be that fuel consumption has increased by 15 or even 20%. With the old BST I could easily do close to 200 kms without hitting reserve - on tar, normal riding at an average speed of 110 kms/h - or, offroad, depending on how technical the tracks were, about 150.
    Now I struggle to get over 160 kms on road and 120 offroad, and the places I normally ride in are quite similar.
    Spark plug is new and so is the air filter. Performance seems ok, the bikes has plenty of power and reaches quickly its top speed. However, it seems not so eager to hit the higher revs as before, although low & mid range power, the ones I normally use, keep being the same as before if not better.

    What may have caused the brownish discoloration of the needle?
  12. spyderman

    spyderman Adventurer

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    Which direction does the snorkel point on a stock 09 DR if forward is considered 12 o'clock. Thanks
  13. ER70S-2

    ER70S-2 Long timer

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    It will only fit one way, straight left: 9 o'clock. :freaky
  14. spyderman

    spyderman Adventurer

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    . Thanks.
  15. bmwktmbill

    bmwktmbill Traveler

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    Meat, gods only know the effect of sucking something into the intake.

    For the screws at the carb bottom a needle nosed vise grips will remove them.
    A little crude but effective.

    Hi Derek.


    b
  16. meat popsicle

    meat popsicle Ignostic

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    Thankfully it was only paper towel, but I don't think I left it in there.

    So I drove up to get jets from Motolab - thanks for letting me in Derek - came home and started the bike to observe the behavior before beginning my investigations and...

    it started (with enrichener) without dying once. Slowly closed the enrichener as the RPMs exceeded 3,000 (occasionally shooting up to 4,000 creates a nice dance) until idling ok and obviously warm. :bluduh

    So its intermittent, but I will begin checking things today.
  17. motolab

    motolab Long timer

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    You're welcome.
    Assuming that you mean the cold start enrichment lever (BST40 carburetors do not have chokes), this means the the mixture is too rich.
    Again, assuming that you mean the cold start enrichment lever, this means the the mixture is too rich.
    Perhaps there were lean areas that were helped by whatever is causing the richness.
    It may be flooding rather than starving.
    Both lean and rich issues can cause high fuel consumption, but your description of how it cold starts points to a rich condition.
    The mixture can be quite rich before symptoms appear that can easily be felt, but there will be a loss of power if it is too rich. As I said, it is possible that the extra fuel from whatever the problem is is helping out some area that is jetted to lean under normal circumstances.

    Start with the basics, which includes checking the float system as previously described.

    Make sure no jets have fallen out.

    Check to see that the cold start enrichment plunger is returning fully when the lever is all the way to the right.

    There are six depressions in the face of the slide guide. Four of these are round. If the round ones are no longer visible, there is no need to look any further, the slide guide and emulsion tube are 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 between .010" (.25 mm) depth and flush, and causes overly rich mixtures in the lower portion of the rpm range. See http://www.moto-lab.com/gallery/view?itemid=298 & http://www.moto-lab.com/gallery/view?itemid=299 vs. http://www.moto-lab.com/gallery/view?itemid=300 & http://www.moto-lab.com/gallery/view?itemid=301. Replace the slide guide and emulsion tube if the slide guide is worn more than .010" (.25 mm).

    Replace the slide if there is any vertical grooving on the downstream side. See http://www.moto-lab.com/gallery/view?itemid=302 vs. http://www.moto-lab.com/gallery/view?itemid=304. If the slide lift holes have been enlarged, replace the slide as a matter of course.

    Replace the slide spring has been clipped (one end of the windings will be open).

    Once you have the fundamentals in place, we can turn our attention to jetting it properly.
    Deposits from ethanol or from poor intake valve sealing.

    Regards,

    Derek
  18. motolab

    motolab Long timer

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    Hello Bill,

    Is it possible that you mean vise grips?

    Regards,

    Derek
  19. ER70S-2

    ER70S-2 Long timer

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    :freaky

    [​IMG]
  20. bmwktmbill

    bmwktmbill Traveler

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    Ummm, yes!

    Just got home last night from a 5K mile round trip to Nevada and back to Wisconsin, my brain is far from it's normal frequency.

    Do you sell replacement screws for the bowl with allen heads?
    Guess that's how I set my carb up.

    bill