see if your idle stop solenoid is hooked up and working .. it is there to idle the engine up with the car running then when you shut the switch off the solenoid kicks out and drops the throttle to shut position .. it may have went bad in the past and a previous owner just set the carb idle higher .. with the idle solenoid you dont adjust the carb idle set screw .,, if you dont have an idle solenoid you can add one ,, i had to put one on my Chevy V6 pinto for the same reason ..