308 idle screw adjustment

Discussion in 'Technical Q&A' started by Gianluca, Apr 14, 2004.

  1. Gianluca

    Gianluca Formula Junior

    May 6, 2003
    Centreville, Virgini
    Full Name:
    Gianluca Chegai

    There are 2 screws on the throttle body on the '81 308: 1 is a large Allen bolt coming diagonally out on the right (standing behind the car) and the other is a small slotted screw in a recess on the left hand side. They both seem to affect the idle. Can anyone tell me about the proper procedure to set the idle and also the reason of the 2 screws?

    Also, I can hear a noise that seems to come from the exhaust. It sound like a muffled but very "tinny" ping; almost as if someone was hitting a very tight tambourine. The sound is fairly random and it comes (or it is more pronounced) as the car gets warm. Any ideas?

    Thank you,

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  3. Smiles

    Smiles Moderator
    Moderator Owner

    Nov 20, 2003
    Pittsburgh, PA
    Full Name:
    Matt F
    From memory, I believe that the large bolt controls idle speed and the slot screw controls mixture richness, but I may have that backwards. I'm sure someone will step in and correct me.

    You may have an exhaust leak. Does it seem to be coming from the engine, or perhaps from where the headers meet the block?
  4. Gianluca

    Gianluca Formula Junior

    May 6, 2003
    Centreville, Virgini
    Full Name:
    Gianluca Chegai
    Thanks Smiles.

    Do you have any idea on how to adjust the mixture screw? On model aeroplanes engines I adjust the idle mixture by opening the throttle fast, to the max, and check for istantaneous high revs. Any correlations?

    I think I must have a small exhaust leak. The sound is not coming from the engine and the exhaust manifold were just reinstalled but the bolts were torqued down pretty snugly and the gaskets coated with copper spray.

    I had 1 reluctant cat-to-manifold joint, however. Probably is that.

  5. yelcab

    yelcab F1 World Champ

    Nov 29, 2001
    San Carlos, CA
    Full Name:
    Mitchell Le
    The official method is to adjust with an CO meter.
  6. seschroeder

    seschroeder Formula Junior

    Apr 25, 2002
    Alexandria, VA
    Full Name:
    Steve Schroeder
    I believe that the small screw sets the throttle plate position while the large "allen" sets the idle speed (1,000 +/- 100). I don't believe that either actual affect mixture as that is done at the fuel distributor.
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  8. mondial86

    mondial86 Formula Junior

    Nov 1, 2003
    Full Name:
    David Holmes
    I am not sure if it is the same for the 81 ,but for 85 ,86 you start with a fully warm motor and turn in the big bolt or air screw,untill it stops ,if the car starts to stall turn in the small throtle stop screw to keep motor runing ,when the air screw is in all the way adj. idle stop screw to give you 800 rpm then open the air screw to goive you 1000 rpm.Hope this helps.
  9. Gianluca

    Gianluca Formula Junior

    May 6, 2003
    Centreville, Virgini
    Full Name:
    Gianluca Chegai
    Thank you all.

    I am taking the car to a mechanic to set the mixture with a gas analyzer.
    just wanted to set the idle correctly.

    Mondial86 will try your method as I read something like you say in the archives but not as detailed as you did,

  10. peterp

    peterp F1 Veteran

    Aug 31, 2002
    Full Name:
    My recap of the procedure is in the old Fchat archives:

    The original procedure was provided by Kurt Kjelgaard -- same procedure as mondial86 described, but the recommended RPM was 450 rather than 800. I set my 86 3.2 this way and it idled much better (yes, believe it or not, the engine will idle at 450).
  11. 4returbo

    4returbo Karting

    Oct 14, 2003
    Full Name:
    Rick Oliver
    I am pretty sure the small screw under is for throtle linkage adjustment. Turn it and notice the trottle cable will slack or tighten. Idle speed can be adjusted with the large hex, but fuel mix is done at the the fuel injection unit.
    I'm just going off what I remember reading in my manual.
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