lc4 enduro bogging after jumps?

Discussion in 'Thumpers' started by j0ney3, Feb 17, 2005.

  1. j0ney3

    j0ney3 Been here awhile

    Joined:
    Feb 12, 2004
    Oddometer:
    315
    Location:
    South Fl0rida
    Hey, I just got a 2001 lc4 enduro w/ 1000 miles on it. Stock everything, I think. Anyway, whenever I hit a jump, large bump, or some whoopdies, my bike bogs down like it's gonna die, sometimes it does die. If it doesn't die, it takes about 30 seconds of throttle fussing to bring it back to normal(throttle fussing=not riding, just keepin the bike running). The only thing I could think of was maybe the PO chopped those springs inside the carb, but I haven't taken that thing apart yet. Any ideas?
    #1
  2. cjflyfisher

    cjflyfisher cjflyfisher

    Joined:
    Jun 5, 2004
    Oddometer:
    414
    Location:
    colorado springs CO
    Sounds like all the fuel is leaving the bowl on a jump. I'd pull the bowl off and see if everything looks normal first. Maybe your missing a splash gaurd or similar gadget.

    Need 640 carb specialist to chime in here
    #2
  3. j0ney3

    j0ney3 Been here awhile

    Joined:
    Feb 12, 2004
    Oddometer:
    315
    Location:
    South Fl0rida
    hrm, wouldn't know what I was looking for...paging Dr. Creeper...
    #3
  4. ChrisC

    ChrisC Amal sex?

    Joined:
    Feb 25, 2003
    Oddometer:
    6,317
    Location:
    Prescott, Arizona USA Earth
    Creep's still in bed...

    This sounds like more than the dreaded BST "slide bounce", that is unless the previous owner really overdid the spring trimming.

    Does the bike ever manifest the bad behaviour at other times, or is it a whoop/jump deal exclusively?

    CC
    #4
  5. j0ney3

    j0ney3 Been here awhile

    Joined:
    Feb 12, 2004
    Oddometer:
    315
    Location:
    South Fl0rida
    Well, it doesn't really like to be bogged down in heavy sand, but it clears out as soon as i can get the revs up. But, no , it only happens on big whoops and jumps.
    #5
  6. ChrisC

    ChrisC Amal sex?

    Joined:
    Feb 25, 2003
    Oddometer:
    6,317
    Location:
    Prescott, Arizona USA Earth
    It shouldn't load up in that situation either. Maybe a good first step would be to take the 17 mm plug out of the bottom of the float bowl. Then pull the main jet (don't lose the little brass collar that surrounds it). The OEM jet was a 142.5. If it's bigger, that'll be a hint if the carb has been diddled with, and you can also check for contamination of the fuel... :thumb

    CC
    #6
  7. ChrisC

    ChrisC Amal sex?

    Joined:
    Feb 25, 2003
    Oddometer:
    6,317
    Location:
    Prescott, Arizona USA Earth
    Oh....and as an aside, when it's "bogged out", if you pull the enrichening (choke) lever does it get better or worse?

    If it gets better you're likely looking at a restriction in fuel flow.

    If it gets worse then it's too fat....maybe a float issue?
    #7
  8. j0ney3

    j0ney3 Been here awhile

    Joined:
    Feb 12, 2004
    Oddometer:
    315
    Location:
    South Fl0rida
    sweet, i'll check tomorrow...is there anywhere i can find some pics of this BST thing that everyone keeps talking about?

    awesome, i'll try that choke manuver this weekend. This whole "carb" thing is messin me up(just sold my r11gs)
    #8
  9. ChrisC

    ChrisC Amal sex?

    Joined:
    Feb 25, 2003
    Oddometer:
    6,317
    Location:
    Prescott, Arizona USA Earth
    Try this:

    BST parts

    It's from H&H's (Georgia) parts department (online). Really helpful site...check it out :thumb

    CC

    p.s. you can now consider yourself a noviciate in the Cult of the BST... :freaky
    #9
  10. cbob

    cbob Been here awhile

    Joined:
    Oct 15, 2004
    Oddometer:
    205
    Location:
    Buford, GA
    Check the needle clip position. Stock is middle, some recommend moving to 4th from top (richer). If you move it one more position you emulate the Factory Pro needle. This should not be a problem with whoops and jumps.


    Check to see if the slide has been drilled. I don't know original size holes, but the recommendation is to drill them to 1/8". This would cause the slide to respond faster to change, like sudden weight changes (G outs) to the slide. Flanny cautioned that drilling the slide can cause bogging in whoops and jump landings. Your condition sounds severe if it takes 30 sec to clear out. If the slide is drilled you can epoxy one hole and drill a small hole next to it. Remember, the other hole is still 1/8" so you need a total area equal to the original holes (which I don't have size info for).

    There is no dampening with the carb slide, so the carb could be flooding after the slide spring rebounds (tops out after bottoming out). With a weaker spring (2 or 4 coils being clipped) it would not push down on the slide as much.

    Like Chris C says, float bowl should probably be checked.

    Let us know what you find, for the sake of the BST cult following.

    - c
    #10
  11. neduro

    neduro Addict Super Moderator

    Joined:
    Jul 8, 2003
    Oddometer:
    12,179
    Location:
    Colorado Springs, CO
    I am not an expert with that particular carb, but have had a similar experience from two different causes:

    1) Float height too high, or lower limit to float travel too low. If there's too much fuel in the bowl, and then the force of the landing pulls the floats down and lets more in, that can explain it- lower the float height. Also, on XR carbs, you can limit how far down the floats are able to travel- if they can go all the way to the bottom of the bowl, you can get bad carburation even with proper float height.

    2) Insufficient carb venting- try putting a "T" in on the breathers, as close to the carb as practical, and running the new line straight up to under the tank. If the breather is filled with fuel, the bowl can't breath and you'll get a weird vacuum situation. Can't explain the physics but can tell you it matters to get it right.
    #11
  12. creeper

    creeper Still alive...

    Joined:
    Dec 13, 2003
    Oddometer:
    10,718
    Location:
    Puget Sound
    Before any armchair (computer desk) diagnostics, it would be good to know what you have for jetting and modifications if any.

    For some unknown reason, I have yet to measure the free length of a vacuum piston spring, so I don't have a reference for you there. You can look to see if it has been unceremoniously snipped however, as opposed to a clean machine made end.

    Next is jetting and needle clip position, followed by measuring the size of the transfer ports in the slide, found on either side of the hole the needle goes thru.
    While your there, check:
    The order of the various washers and spacers on the needle, when compared to an exploded drawing.
    The condition of the vacuum diaphragm. Holes, pinch marks and the like.

    The vents previously mentioned by my fellow armchair carb experts, and the vent under the diaphragm as well (see item circled)

    [​IMG]

    Has the snorkle atop the airbox been removed?
    Is there a high flow side cover?
    Does fuel flow well from the petcock?

    And last but not least, I was given a spec for a float level wet test of 0 to 2mm below the center of the float bowl gasket surface at the center of the float bowl.
    I have not tested this to compare it to a known correct float level... so it could an inaccurate figure.

    How to perform a "wet test" you ask :dunno
    Replace the black bowl drain hose with a clear one and tape it to the carburetor so that it is centered on the bowl at the bowl / carb body gasket surface.
    Level your bike, open the drain screw (see item circled) You should see your bowl fuel level in the hose. You can also start your bike and watch the "running" fuel level.

    Thats all fer' now :wave
    Chris
    #12
  13. ChrisC

    ChrisC Amal sex?

    Joined:
    Feb 25, 2003
    Oddometer:
    6,317
    Location:
    Prescott, Arizona USA Earth
    'Bout time you got out of bed. And why do you have a Pet Cock? :wink:
    #13
  14. creeper

    creeper Still alive...

    Joined:
    Dec 13, 2003
    Oddometer:
    10,718
    Location:
    Puget Sound
    That all you got? Man, you is slippin' down the long hill to...
    [​IMG]
    #14
  15. j0ney3

    j0ney3 Been here awhile

    Joined:
    Feb 12, 2004
    Oddometer:
    315
    Location:
    South Fl0rida
    yes.
    no.
    I don't know what flowing well should be.

    BST cult, holy shit, is there anyone who rides an LC4 who is even remotely sane? Is the LC4 the culprit of said insanity, is the BST? I guess I'll rip apart the BST this weekend and take some pics...what have I done?
    #15
  16. ChrisC

    ChrisC Amal sex?

    Joined:
    Feb 25, 2003
    Oddometer:
    6,317
    Location:
    Prescott, Arizona USA Earth
    Yes, all LC4 pilots are at least semi-psychotic, and no....it has nothing to do with the BST...

    Conventional wisdom here is that the airbox top/snorkel should remain in place and that the KTM "racing" side cover gives the best results for air flow. This has apparently been verified by Sommer (Euro KTM mavens), so it's now part of the BST lore.

    Fuel flow from the petcock should be brisk, say 1/2 litre per 30 seconds. Low flow would certainly contribute to the symptom you describe

    Good luck...

    CC
    #16
  17. dagwood

    dagwood Banned

    Joined:
    Aug 25, 2003
    Oddometer:
    35,427
    Location:
    Vancouver, Not! BC...
    no
    #17
  18. creeper

    creeper Still alive...

    Joined:
    Dec 13, 2003
    Oddometer:
    10,718
    Location:
    Puget Sound
    In theory, the CV carb (in this case the holy icon Mikuni BST :pope) works best when there is small degree of negative pressure in the airbox, tests by the Techno-Germanic BST tribes ( warm beer swilling zelots :freaky ) have shown this to be the case.
    Please refer to the 5th "book of the BST", chapter 39, verse 16 :deal ... "and yea, tho I :ricky thru the schmutz and bad juju, I shall fear no evil, for the negative pressure is with me".

    So sayeth the one...
    #18
  19. ChrisC

    ChrisC Amal sex?

    Joined:
    Feb 25, 2003
    Oddometer:
    6,317
    Location:
    Prescott, Arizona USA Earth

    Unter Druck über allen
    #19
  20. Esteban

    Esteban Banned

    Joined:
    Jun 7, 2004
    Oddometer:
    5,799
    Location:
    Above Golden
    It's the CV carb. Avoid whoops, jumps, bumps and other obstacles. Or get a non CV carb.

    SG
    #20