Default idle/mixture screw positions

Discussion in '308/328' started by t3thomas, Jun 1, 2009.

  1. t3thomas

    t3thomas Formula Junior

    May 9, 2008
    North Carolina
    Full Name:
    Long story short, I screwed-up and didn't mark the prior positions before beginning work. Car initially ran okay, but it had a rev surge which I traced to a vacuum leak in the intake plenum manifold. Now that it has been sealed the car won't start. To rule out all of the other posibilities, sensors are new and traced correct, sparks on both banks, fuel pump running as req'd. (safety switch disconnected, etc.), battery new and fully charged, AAV operating correctly.

    When I've tried w/ everything hooked-up correctly the engine sputters but won't start. When the plenum hose is disconnected or a plug removed, there is a lot of fuel so I'm assuming flooded by the cold start injector. Disconnected CSI and still won't start. I'm thinking the adjustment screws which were initially set when the car was running somehow compensated for the vacuum leak, but now that it has been sealed they need to be somewhat close to their final positions before the car will start. Any suggestions, pictures, etc?
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  3. CliffBeer

    CliffBeer Formula 3

    Apr 3, 2005
    Seattle, Washington
    Full Name:
    For a mixture screw, start with two full rotations out from fully seated - webers designed with this general position in mind. For an air screw start with one full rotation out from fully seated and adjust from there.
  4. Spitfire

    Spitfire Formula 3

    Nov 16, 2006
    Agree with the idle mixture screw baseline setting of two full turns out, but I'm sure I read that the air bypass screws should be fully closed, and that as few of the air bypass screws as possible are unscrewed to achieve balance between barrels. Please refer to Birdman's excellent carb tuning procedure here:
  5. t3thomas

    t3thomas Formula Junior

    May 9, 2008
    North Carolina
    Full Name:
    Thanks for the responses, however I've got an '84 QV. I found a thread from several years ago searching for 'idle adjustment' and it appears that for the idle adjustment screw.........."place a piece of paper between the screw and tab on the plenum butterfly and tighten the screw just until the piece of paper can be pulled out then tighten another 1/2 turn." It then goes on to say ......."the air bypass screw should be fully seated, then backed out 4 full turns".

    I haven't tried this default position yet, but it sounds reasonable. I believe if the car will start from this point, the plenum screw should then be used to 'fine-tune' the idle to near 1000 rpm?
  6. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    North shore, MA
    Full Name:
    THE Birdman
    The butterfly stop adjustment/air bypass at the throttle body should not keep the car from starting. It might idle high or low, but all those do is set the minimum airflow, not the mixture. The mixture is adjusted deep within the CIS system.
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  8. treventotto

    treventotto Formula Junior

    Apr 14, 2008
    Full Name:
    Got any pictures of that? so funny...
  9. gilligan308

    gilligan308 Karting

    Dec 8, 2008
    Full Name:
    Do you think you flooded the engine by pulling the safety wire on the fuel dist. As soon as you let it run it is spraying into the runners. I flooded mine a couple of weeks ago when i was diagnosing a bad accumulator. Put your pedal to the floor and let it breathe and burn up some gas aand hope fully it will fire.The accumulator will also keep it from starting.

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