Walbro carb on 66cc China Girl

Discussion in 'Performance Mods' started by Arty, Jun 29, 2015.

  1. Arty

    Arty Active Member

    Enclosed are a few pictures of the mods I did to adapt the JN Motors Stage 3 Walbro Carb kit to my 66cc China Girl engine.
    The engine has 40mm stud spacing, not the 32mm provided with the stage 3 kit.
    I also wanted to incorporate a thermal isolation spacer I fabricated from 1/2inch thick UHMW (ultra high molecular weight) material. Since I couldn't find long enough metric allen head bolts to reach all the way through the velocity stack, carb, thermal isolation spacer etc., and finally into the 40mm RSE Reed Valve, I built a reed block adapter incorporating a throttle cable mount, threaded to accept the velocity stack/carb mounting bolts provided with the kit, The adapter mounts between the carb, and the isolation spacer using the bolts provided in the kit. Thus, you bolt the stack/carb assembly to the adapter, then bolt the adapter to the reed block, with the isolation spacer sandwiched in between, using easily available longer allen head bolts.
    I then modified the carb throttle shaft eccentric to incorporate a swivelling throttle cable clamp, with the new adapter plate built with the throttle cable mount positioned in such a way as to provide wide open throttle, since the cable adapter supplied with the kit was oriented in such a way as to provide only about 3/4 full throttle. The adapter plate also incorporates a stand off for mounting an additional throttle return spring, since I didn't trust the little helical spring provided on the carb.
    I also drilled and tapped the carb diaphram cover plate to accept a hose barb for the vacuum impulse provided from my crankcase vacuum line (also drilled and tapped for a hose barb), since I noticed that the little vacuum hole provided on the back mounting face of the carb would be blocked when the carb was installed.
    You might also note that I port matched the thermal isolation spacer such that its back opening is machined with a rectangular shape to match the inlet of the reed valve, and the front face is round, to match the carb.
    I have to say, that after about two days of tuning, I am really happy with the set-up. It seems to have enhanced all aspects of the performance. It has good punch off the line, really good mid-range, and better top end.
    The only thing I am struggling with now, is that I can't seem to get it to idle down as it should. - - Any suggestions?

    Attached Files:

    Last edited by a moderator: Dec 15, 2015

  2. jaguar

    jaguar Well-Known Member

    now all you need is a good exhaust pipe
  3. The_Aleman

    The_Aleman Active Member

    That looks pretty sweet!
  4. KCvale

    KCvale Motorized Bicycle Vendor

    Nice build!
    How does the carb perform?
    Any noticeable performance gain?
  5. Arty

    Arty Active Member

    You're right about the pipe. - Any suggestions?
    By the way - it's also running one of your ignitions - and I'm going to play with the timing now that the new carb is on. - The engine has a few other mods as well - transfer port reversion mod - shaved cylinder - Fred Head etc..
  6. Arty

    Arty Active Member

    I'm actually really happy with the new set-up. - It feels stronger throughout the whole range, and has really nice throttle response. It's a 7 speed, and now it really pulls strong in 7th, so keeping up with city traffic is no problem, and you can pull the front wheel in 1st if you abuse it a bit - I try not to.
    So far though, I can't get it to idle down. I'm trying hard to discover if I have a vacuum leak somewhere to see if that's the problem.
  7. KCvale

    KCvale Motorized Bicycle Vendor

    Don't ya just love that?
    I like 3-speed internals myself but the overall gear range is the same as a 7-speed, just less steps in between.
    Pulling high gear over 35mph is all grunt power so it's nice to have ;-}

    Pop off your air cleaner and verify the throttle cable is letting the throttle fully close down to the idle set screw.

    I always have to unscrew the throttle stop and remove it's set screw on the engine and screw it back all the way in for enough cable slack to get idle, but this is with the stock carb.

    Again, nice build.
Similar Threads -
  1. Arty
  2. dchevygod
  3. Kit
  4. Firestorm102389
  5. bahramu