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Thread: MFI troubleshooting - high idle, pops and bucks at part throttle

  1. #11
    Pull your spark plugs see if they are black or white (rich or lean) Ideally you will have an AFR meter, but it can done without, just takes longer. If the plugs look lean, which I think they will be, go two clks richer on the main rack adjustment (counterclockwise) if car can be driven, accelerate slowly but with steady throttle opening in second gear going slowly up to 5000 RPM. Assuming it is better but still not right repeat two clks richer adjustment and repeat test. If you are going in the right direction the performance will be getting better not worse. Ultimately your goal is to be able repeat this acceleration test with the engine pulling smoothly up to 5000. Generally when I see these cars someone has gone way to rich. In that case I will keep leaning rack two clks at a time until I know it has gone too lean, then two clks at a time richen until that acceleration curve is smooth. Then that last adjustment is the idle adjustment, which without a meter you will simply have to do by ear, feel, and idle speed. I use an AFR meter of course, but I could get very close by this seat of the pants method.

    By the way, generally 72-73 throttle bodies were redesigned from the previous ones that wore prematurely and so didn't wear out as fast.
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  2. #12
    Scope Creep Poster Child
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    Arne-

    Is there play in the throttle shafts when you try to move them up and down?
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  3. #13
    Serial old car rescuer Arne's Avatar
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    Quote Originally Posted by Scott Clarke View Post
    Arne-

    Is there play in the throttle shafts when you try to move them up and down?
    Nope. As I noted earlier, that was one of the first things I looked at when I started this, due to the high idle speed, now fixed.
    - Arne

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  4. #14
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    Arne -
    Not sure if this will help u but lots of decent research on the bird from back in the day. I went through it when messing with our MFI and some was helpful.

    https://www.pelicanparts.com/techart...MFI/TipMFI.htm
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  5. #15
    Senior Member 2.5MFI's Avatar
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    Ed Mayo had some great information. I would add, not to play with the 114mm rod.
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  6. #16
    Serial old car rescuer Arne's Avatar
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    Quote Originally Posted by Scott72 View Post
    Arne -
    Not sure if this will help u but lots of decent research on the bird from back in the day. I went through it when messing with our MFI and some was helpful.

    https://www.pelicanparts.com/techart...MFI/TipMFI.htm
    I had seen that page, and downloaded the PDFs from there. Definitely worth reading.

    Quote Originally Posted by 2.5MFI View Post
    Ed Mayo had some great information. I would add, not to play with the 114mm rod.
    Thanks, Mark. I gathered that I should leave the pump pushrod alone, and move on to other adjustments.

    I am still a touch concerned with the popping at idle. Some of the info on the above linked Bird page seems to indicate that this popping at idle may be due to slight mis-adjustments in either throttle plates or air screws. Should I try to eliminate that before moving to the main rack adjustment?
    - Arne

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  7. #17
    You can try resetting just the idle circuit adjustment, but remember, adjustment of the main rack affects the entire range, an idle adjustment only affects idle and slowly phases out around 2-3000 rpm. That is why you try to make main rack adjustments first, then finish with idle. Hard to describe feel and experience via cyber space. When you said you measured the pump rod at 114 I assume you meant center to center,,,,not overall. Measure overall, then measure o.d. diameter of socket, then subtract from overall, that will be center to center.
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  8. #18
    Serial old car rescuer Arne's Avatar
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    Thanks, Ed. Yes, the 114mm was measured center-to-center. I have the old Lee Rice articles that also gave that method for determining the length.

    I don't think I want to go out of order on the adjustments, will do the main rack first and only go to the idle circuit once I'm happy with the main rack setting.

    Just unsure on the popping at idle. Is that from mis-adjusted throttle plates, or pump adjustments? Obviously no one can make that determination over the internet...

    Might try to further fine-tune the air screws to reduce popping before moving to the main rack adjustment.
    - Arne

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  9. #19
    Serial old car rescuer Arne's Avatar
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    Pulled the plugs this morning to verify where I was prior to going further. Two surprises found. The minor surprise is that the plugs are NGK, but not the BPR6ES (or iridium equivalent) I was expecting, but BPR7ES which is one step colder than I had guessed for this car. EDIT - Looks like BPR7ES may be correct for a 2.4T?

    The bigger surprise is that all 6 plugs (gapped at .032") are black and sooty - dry fluffy soot, not wet or oily. Not what I was expecting at all. EDIT 2 - Info I have is that plug gap should be .024", so I will probably want to adjust that.

    So despite my gut feel, it appears to be running rich, not lean.

    Stopping here to wait for other opinions.
    Last edited by Arne; 03-30-2019 at 10:59 AM.
    - Arne

    In Progress
    - 1972 911T coupe, born (and someday will be again) Silver
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  10. #20
    I use the BPR7ES for all my 'S' cars, they should work fine even for a 'T' but a BPR6ES would be a better choice for a 'T'. That was why I always start by looking at spark plugs, gives you a better idea of where to go. I would start now by leaning the idle adjustment only, we know the main rack is likely too rich already. The idle adjustment is the lower pump adjustment , reached thru the fan blade and the hole in the shroud. Push in and turn until you feel the tang drop in the slot, then turn clockwise to go leaner. Go about 4 clks at a time. If leaning the idle seems to help you may need to go leaner on the rack since that leans out the whole system otherwise the idle adjuster may run out of adjustment range. And yes, I use the .024" gap.
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