... ....Sure sounds like a vacuum leak....Try closing the choke butterfly a litttle as the idle falls off to see if it changes..Power brakes.??.....Pinch off the vacuum hose to the booster with vise grips....Automatic transmission.??.....Block off the line to the modulator.......Same goes for the vacuum advance unit.....Intake gaskets.??....carb mounting gasket.??.....carb mounting gaskets can be tricky...Lay the gasket on the intake and make sure it really is sealing around the bores.....make sure the gasket is sealing on the carb too...like the port for the booster...