LC4 Adventure Carb problems?

Discussion in 'Thumpers' started by ktmnate, Dec 9, 2004.

  1. clintnz

    clintnz Trans-Global Chook Chaser

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    May 17, 2004
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    A test bench even! Do you know that you are supplying the same vacuum that the carb sees under load? How is the diaphragm?

    Somewhere in the BST40 thread there is info on how to get a measure on slide & guide wear, the depth of the dimples will tell. They don't all wear out at as quick as some retailers of carb parts might have you believe, but if they are getting shagged maybe best replace them given the nature of the problem.

    Cheers
    Clint
    #41
  2. Padowan

    Padowan Adventurer

    Joined:
    Feb 26, 2012
    Oddometer:
    10
    Location:
    South West UK
    No, I can't tell that the vacuum is the same, but i did some cursory estimations. A 650cc engine running at 5,500 rpm is drawing in around 65 cubic feet a minute of air, which most decent vacuum cleaners should be able to shift (they reckon a decent vac will pull 80-100).

    The guide is worn down past the wear marks, so it's definitely part is best, the slide is showing some scoring, and both the needle and the emulsion tube are showing some signs of wear. I will replace all those components, and test again.

    Sent from my SM-A520F using Tapatalk
    #42
  3. Padowan

    Padowan Adventurer

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    Feb 26, 2012
    Oddometer:
    10
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    Well, replacing the slide, the guide, the emulsion tube and the needle hasn't fixed the problem.

    On the vacuum cleaner the slide moved further than before, but still not to the top - it's not sticking, it just seems to reach equilibrium part way through it's motion.

    I have basically run out of things to check now. :(
    • On the carb
      • I have replaced the components as above
      • Diaphragm is not perforated
      • Float level has been set
      • Jets are clear and appropriate for the pipe/airbox and confirmed by AFR readings being acceptable
    • Other checks
      • Octane selector correctly earthed
      • Fuel delivery to the carb is fine
      • Valve clearances are fine
      • Air filter is clean and not over-oiled
      • Plug looks fine, normal tan colour
    I either need inspiration, or need to accept that the bike won't ever go above 70mph.
    #43
  4. motolab

    motolab Long timer

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    6,283
    Does the bike have the EPC installed?

    BTW, "AFR" readings don't confirm anything.

    Regards,

    Derek
    #44
  5. Padowan

    Padowan Adventurer

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    Feb 26, 2012
    Oddometer:
    10
    Location:
    South West UK
    The EPC system is removed and the spigot is capped and sealed.
    #45
  6. motolab

    motolab Long timer

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    Back to the drawing board...

    Regards,

    Derek
    #46
  7. Padowan

    Padowan Adventurer

    Joined:
    Feb 26, 2012
    Oddometer:
    10
    Location:
    South West UK
    So I tried another test today, I connected a hose to the atmosphere vent that I could blow into to increase the pressure below the diaphragm manually. Doing this with the engine off you can clearly hear the slide returning when you stop blowing and when the engine is idling, the bike responds like if you put the choke on, you can hear the engine bogging as you raise the needle and richen things up.

    I got the bike up to the limiting speed in top gear and blew into this tube in the hope that it would move the slide (if it was pressure/vacuum limited in it's movement) upwards and I would feel a burst of acceleration/power. But nothing, no change, was going red in the face blowing so hard, barreling down the road at about 65mph with one hand holding the throttle on the end stop and the other holding the tube in my mouth! So from this test, I can only assume that the problem isn't being caused by a limit in the movement of the slide, so I think it's back to square one for the investigations.

    Apart from this top-end limit in speed the bike performs excellently, it's punchy low down, idles and cruises smoothly, it's only when you're giving it a fistful that you would even realize that something isn't quite right.
    #47
  8. gunnerbuck

    gunnerbuck Island Hopper

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    Try going down one jet size and then up one jet size on the main and see if it effects the response as your problem is in the top throttle range ... If nothing changes then I would have a look at your valves for wear at the seat/valve interface..
    #48