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Thread: MFI Backfiring

  1. 07-10-2011, 05:16 AM #1
    50/50
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    MFI Backfiring

    All
    I have a 71S MFI with a Permatune Ignition,that developed a sudden severe backfire/popping on acceleration after the engine had been fully warmed up. Initial diagnosis was a high resistance connection on one of the spark plug connectors.
    It ran great from cold for a while then started the popping on acceleration. Next diagnosis was the MFI was set too rich to compensate for an inoperative cold startup switch. Then to correct we disconnected the MFI linkage and reset the system to factory spec.
    After letting the engine warm up for 5+ minutes on an 80dg day ,it lightly pops when accelerating for a while. Then it stops and but now does the same when deaccelerating.
    Any ideas?
    Thanks,
    50/50
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  2. 07-10-2011, 09:06 AM #2
    Flieger
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    Max Sluiter Flieger's Avatar
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    Backfiring on deceleration might be the RPM transducer or another part of the speed switch which shuts off fuel on the overrun.
    1971 911S, 2.7RS spec MFI engine, suspension mods, lightened
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  3. 07-10-2011, 04:25 PM #3
    edmayo
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    So when did it start these antics, after the Permatune install or after you changed the MFI settings? Did you also install a replacement Bosch silver coil?
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  4. 07-11-2011, 01:21 AM #4
    50/50
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    Hello Ed
    The Permatune was installed by the PO and it ran fine with it. The coil has also been on the car for a while. The resetting of the MFI was done recently.
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    50/50
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  5. 07-11-2011, 08:18 PM #5
    edmayo
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    What exactly did you reset on the MFI?
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