Total loss of WOT/top end power.

Discussion in 'Crazy-Awesome almost Dakar racers (950/990cc)' started by Jdeks, Jan 12, 2013.

  1. Jdeks

    Jdeks Departed

    Joined:
    Aug 30, 2009
    Oddometer:
    1,027
    Okay, so I've just done my valves and installed pyndons prefilter, and configured my carbs as per below:


    42 pilots
    158 front main
    162 rear main
    IMS 1.5 turns front, a.625 turns rear
    stock needles, thrid from top
    floats at 3mm
    all else stock

    My SAS and EPC are gone, and I have a 2-1 exhaust with a 'straight-thru' style pipe (Barrett). Carbs are synced, TPS calibrated.

    Since putting it all back together, I no longer have enough top end power to even get to 100 kph. The very first little jump when you hit the throttle low in the revs is there, but it dies instantly, and even on full throttle, acceleration is like that of a 250cc.

    I'm following the carb tuning advice on the HOW, which seems to say my symptoms indicate my mains are too small. Pulled the brand new rear plug, very end of the tip is white, which I take means lean. So I;ve stepped the mains up to 162 front 167 rear, then 167 front 172 rear. Slight improvement each time, but not much, reluctant to go any larger. Also, she snaps, crackles and pops like a bitch on deceleration.

    Where should I look for what I've done wrong? Could this be the result of botched valve clearances, or is it going to be carbs? If so, which part?

    Thanks heaps for any help.

    PS I found the original working carb settings: 155 rear main, 150 front main, factorypro needles on 3rd clip, front and rear IMS at 1.5 turns, floats at 2mm
    #1
  2. AdvRonski

    AdvRonski They call me......Ronski

    Joined:
    Feb 4, 2007
    Oddometer:
    722
    Location:
    Wishing I was back in Grand Junction, now in MN
    Look for a fuel delivery problem- plugged filter, pinched fuel or vent hoses, failing fuel pump.
    #2
  3. AUT-EasyRider

    AUT-EasyRider Been here awhile

    Joined:
    Aug 7, 2010
    Oddometer:
    127
    Location:
    Austria
    Maybe just the fuel line is squeezed when you mounted the tank, happend to me, same symtons.
    #3
  4. Jdeks

    Jdeks Departed

    Joined:
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    Hey guys, thanks for the suggestions. Checked the fuel lines, all good - but by pure luck it put me onto the actual problem.

    I hadn't seated the seals under the black plastic end caps properly - the little copper tube was crooked on both of them. So the slides were moving back and forth WAY too easyilty (no suction noise/resistance when i played with them). Re-seated both seals, running fine.

    My knowledge of carbs is limited - if someone can chime in and tell me what effect that would actually have when running the engine, that would be great. My theory is that when I hit the gas, the slides weren't opening (no suction in the chamber behind the black end caps), and so I had bugger all flow into the cylinders.
    #4
  5. spafxer

    spafxer Long timer

    Joined:
    Mar 12, 2007
    Oddometer:
    2,658
    The carburetors are called Constant Velocity Carbs. I just found this simple YouTube video on how they work..

    http://www.youtube.com/watch?v=wyspAHrMbb8

    Sorry, I don't know how to embed that.

    They are marvelous at providing good smooth throttle response and compensate for altitude mixture adjustments automatically.. :clap


    .
    #5
  6. Jdeks

    Jdeks Departed

    Joined:
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    Hey, thanks for the video, most educative.

    Still having minor problems with the carbs, though, and my knowledge isn't quite enough to know where to go

    Pulls hard as heck from 7500 rpm + ,literally gives a yank spot on 7500. So I think I have the mains right. But the low end doesn't have the kick it should. 155/158 mains, stock needle on third clip, little over 3mm floats., IMS 1.5/1.675,42 pilots. Still running the 2-1 pipe, and pyndons pre filter.
    #6