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308 GTSi base idle

Discussion in 'Technical Q&A' started by rjlloyd, Apr 1, 2020.

  1. rjlloyd

    rjlloyd Formula Junior
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    Jun 19, 2014
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    Hello All
    I have a Euro (UK) 1981 308GTSi, I am looking for the base idle spec. i.e The idle speed set with the throttle plate set screw and the idle air bypass screwed all the way in.

    Many Thanks in advance
     
  2. waymar

    waymar Formula Junior

    Sep 2, 2008
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    FWIW my ‘82 GTSi base idle with full in air screw is set at 1000-1100 using meter not tach. I have euro digiplex, no cats/air pump and euro adjusted ex cam. Just saying....
     
  3. rjlloyd

    rjlloyd Formula Junior
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    Thanks Wayne, so your idle is set purely by the set screw and not using the idle air bypass at all ?
     
  4. waymar

    waymar Formula Junior

    Sep 2, 2008
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    Richard that is correct. I find the idle and low speed operation is better w/o the air bypass screw. Simple adjustment by set screw under the butterfly...... again I use a meter and not just the tach...... my tach reads a tad high. My vin plate reads June 1981 although the papers always had 1982 since birth....
     
  5. thorn

    thorn F1 Rookie
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    This isn't how you attain proper idle speed...
     
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  6. waymar

    waymar Formula Junior

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    please continue your thought process.....
     
  7. Steve Magnusson

    Steve Magnusson F1 World Champ
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    That's the procedure shown in the 281/83 Mondial8/QV WSM, but later procedures for models equipped with Bosch CIS systems (even others with K-Jet without Lambda) show setting a reduced RPM with the air bypass closed and then opening the air bypass to bring it up to the 1000 RPM. It's not like you couldn't try it both ways and see if there's any significant difference in behavior is it?
     
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  8. waymar

    waymar Formula Junior

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    While we are in the topic of the bypass screw. If you can obtain idle without it... what was the designed function? Just asking....
     
  9. thorn

    thorn F1 Rookie
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    The plate set-screw sets the minimum travel return for the throttle plate; which is (sort of relatively) the same thing as simply holding the throttle pedal in a particular position.

    One doesn't set base idle by adjusting the throttle cable's min/max response range. The throttle plate purpose is to modify (ie, increase) the RPM from baseline idle, not determine baseline idle.
     
  10. Steve Magnusson

    Steve Magnusson F1 World Champ
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    Couple of reasons that I can think of (but just speculating):

    1. On systems with multiple throttle bodies that don't have a mechanical way to balance to two banks (i.e., a way to mechanically adjust the two throttle plates relative to each other), it gives a way to balance (equalize) the idle airflow of each bank. I can't recall anyone posting the 512BBi WSM (or I'd have grabbed it ;)), but that would be another interesting reference to check also being a K-Jet without Lambda system (so if someone's got it please post), and

    2. In theory, having the throttle plate opened as little as possible at idle reduces the change in RPM if the mechanical position of the throttle plate varies at the idle position. Having some air flow thru the air bypass requires the throttle plate to be less opened (to keep the same 1000 RPM idle). This graph:
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    from this paper (page 14):

    http://www.diva-portal.org/smash/get/diva2:23184/FULLTEXT01.pdf

    shows how the throttle open area changes versus throttle plate angle. The lower the throttle plate angle, the lower the slope of the curve, so any uncertainty in the throttle plate angle results in a smaller change of the open area.
     
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  11. waymar

    waymar Formula Junior

    Sep 2, 2008
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    Well while the motor was warm after a drive I set the throttle plate screw to 750 rpm and adjusted the air screw to 1050. I will have wait until dead cold to see how it interacts with cold starts.
     
  12. rjlloyd

    rjlloyd Formula Junior
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    After reading another post I was prompted to look at the Mondial 8 WSM and found the following re idle speed. In 30 odd years of being a mechanic I can't recall another vehicle where the base idle was set by the throttle plate set screw with the idle air bypass not used
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  13. Steve Magnusson

    Steve Magnusson F1 World Champ
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    #13 Steve Magnusson, Apr 2, 2020
    Last edited: Apr 2, 2020
    I think you can conclude that the text of that Fig 11 was (wrongly) copied from another application using multiple throttle bodies as there is nothing to "balance" on a Mondial with a single throttle body and single intake plenum.
     
  14. thorn

    thorn F1 Rookie
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    I set the base idle with the air, then blip the throttle and watch the fall-off RPMs to set the throttle. Basically, using the throttle screw to control the drop on decel, so that it doesn't go too far down. I don't want the throttle screw to SET the idle RPM; I want it to keep the throttle falloff from going so far that the engine dies.
     
  15. waymar

    waymar Formula Junior

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    Okay - dug up some notes from +10 yrs back, I believe it was from a dialogue with Fletch when I had my WUR & FD rebuilt. (Set the throttle plate so it just is not touching (.004”/.101mm) he mentioned a thin strip of paper would do. Just do not want the plate to seat. Then set the idle speed with the air idle screw.)

    I removed and cleaned the air idle screw and replaced the rubber ring at the end of the spring for better sealing to the housing and the air screw threads.

    I was able to set the idle at 1000 using a multi meter. Holds stable..... this works for me .... just saying...
     
  16. yelcab

    yelcab F1 Veteran
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    So I now have heard of three methods.

    1. Close the bypass screw, set the idle screw to 500 RPM, open bypass screw to bring idle speed to 1000 RPM
    2. Set idle screw to have a paper thin gap to the throttle lever, set idle speed to 1000 RPM using only the air bypass screw
    3. Set idle speed using air bypass screw, and then supplement that with a bit of idle screw to help in deceleration

    They are drastically different, and cannot all be correct. What gives?
     
  17. rjlloyd

    rjlloyd Formula Junior
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    Typically in my experience
    1. would suit non electronically controlled systems, usually used to to stop "running on"
    2. would be used in a full electronic control system using an idle speed control motor. Throttle stop set to stop the plate binding in the body, but other than that engine idle speed controlled by ECU
    3. Cant think of a car off the top of my head that uses that setup
     
  18. thorn

    thorn F1 Rookie
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    #2 was the first method I tried. The result was close, but not optimal. Therefore, #3 is a step that follows #2 on my car that resulted in my desired idle performance.
     
  19. yelcab

    yelcab F1 Veteran
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    The CIS system being one step above carburetors, it does not have any Idle Control Valve like DME electronics. That would lend itself to method 1) using both the idle screw and bypass screw to set the idle speed.

    I cannot remember where I read of method 2 …
     

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