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vacuum specs for 77 911S

Engine

Vehicle Information: Year: 1977; Body Type: Coupe;

Is there a specification for normal engine vacuum? Also, what would cause a fluctuation in timing mark lineup when using a timing light?My engine produces vacuum at the very bottom of the green scale on a vacuum gauge at idle, with a small amount of flicker. This is with the timing set roughly at Z1, but that also seems to jump around. If I advance the timing I can get the vacuum into the middle of the green range, but I'm not sure what adverse effects that may have over setting at Z1. It does seem to worsen a light throttle shudder that won't seem to go away at any set-up I've tried. Car has 58,000 miles on it.

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