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Thread: Engine rpms oscillating rhytmically

  1. #1

    Engine rpms oscillating rhytmically

    Hello,

    I have an 73 911S and this evening, I suddenly experienced a strange problem:

    The car starts and idles fine (cold and warm), revs up to 1900 rpms normally. Going over this invisible mark, it starts revving/oscillating rhytmically! If I hold the accelerator steady as if i would like to achieve i.e. 2500 rpm, it begins to rev rhytmically: up to 2500, immediately falling down as if it would stall, then catching itself just at idle speed, reving up to 2500 rpm, falling down... this is oscillating up and down, all while I am holding the accelerator steady.

    This means I can only drive with rpms up to 1900 rpm, otherwise it gets to the point of undriveability.

    The same thing happens if I try to accelerate with the hand throttle - everything over 1900, and the engine begins to oscillate. One up/down-cycle last for about 1,5 seconds.

    Any ideas?
    Thanks
    Hannes

  2. #2
    Jared Rundell - Registered User JCR's Avatar
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    Dec 2002
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    Birmingham, MI
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    I suspect fuel delivery or an MFI adjustment (and I certainly don't claim to be an MFI expert).

    But, I had a similar condition to what you describe - it was due to wires in the CDI box that had worn insulation from rubbing against the case. It certainly wasn't the first thing I thought of. The car idled fine, but had fits with anything over 1500 rpm.

    First, I'd check the fuel filter, points gap, and coil.
    Jared
    '73 911S #0793
    '69 912_ #0602
    Early S #0454
    RGruppe #0391

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