[FWB]

if you can verify the vacuum gauge is in correct working order, i would question the needle bouncing, how much is it bouncing, is it more than 1 in vac.your experiencing run on after the key is turned off. where is the timing set? is there fuel dripping into the plenum from the carb without moving the throttle? a fluctuation of 2 or more inches of vacuum can be a sign of valvetrain issues. broken springs or bad valve guides can cause a vacuum reading fluctuation. not trying to be an alarmist, and i have no idea how new or old your build is, but there seems to be more going on than having to adjust the carb.