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Thread: MFI Newbie

  1. 06-05-2019, 04:01 PM #1
    mjk911
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    MFI Newbie

    This will certainly seem a dumb question for you more mechanical-minded guys, but I just migrated from long-term 356 ownership and acquired my first MFI (a '72 911). The dealer I bought it from checked the usual stuff, timing, dwell, etc. and rebalanced the pump, adjusted all the linkages, installed new injectors, etc. etc.
    But it still backfires if you lift off if you are at speed and slowing for a stop.
    I have booked it in to our local MFI guru, but I have to wait a month - my question is this: some of my friends say I should not drive it at all for fear I damage the motor. I hate to waste a month of summer driving (weekends only, no major distances) - should I really park it??

    Thanks!
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  2. 06-05-2019, 04:16 PM #2
    RSTarga
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    He may not have adjusted the fuel cutoff micro-switch properly. Very simple fix you can do yourself.
    That said, sometimes it is due to not removing your foot completely off the accelerator pedal when you lift off.
    David

    '73 S Targa #0830 2.7 MFI rebuilt to RS specs
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  3. 06-05-2019, 04:21 PM #3
    Arne
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    Should not be a problem to drive it while you wait, if you don't want to check this yourself. But it's not a hard system to check out.
    - Arne
    Current - 2018 718 Cayman, Rhodium Silver, PDK

    Sold - 1972 911T coupe, Silver Metallic; 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic
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  4. 06-06-2019, 01:55 AM #4
    frederik
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    You should track down a copy of the CMA (Check-Measure-Adjust) manual from Porsche. It has detailed instructions for checking the cutoff switch and related systems. It's not working on mine either, and we discovered that the RPM transducer (a little metal box next to the ignition box) is not working. I'm going to open it and try to fix it.

    That being said, yesterday my MFI was finally adjusted correctly (it was running too rich and popping a lot) and now I don't get ANY popping or backfires when lifting off, even though the cutoff switch is not currently connected and it's running very rich at that point. So it might be the case that you're also running too rich normally.

    Running too rich will eventually dilute the oil with gasoline so it's definitely something to fix.
    1970 2.2S Elfenbeinweiss
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  5. 06-06-2019, 04:37 AM #5
    72targa
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    https://www.pbase.com/slidevalve911r...docs_bosch_mfi
    Peter Kane

    '72 911S Targa
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  6. 06-06-2019, 12:36 PM #6
    72t4tom
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    welcome to the 6 door club
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