I have a 390, 4V. I checked vacuum ( 20 lbs and steady ). I loosened the distributor to adjust the advance. I great a maximum idle speed, tighten down. Taking car for a test drive, I get great launch but have a loud "chatter". I loosen the distributer, set advance counterclockwise, finally got rid of chatter but performance is poor. I can't seem to get no chatter and good performance... Any ideas? I checked the vacuum line leading to the carb and it is attached to the timed spark vac source ( this is a new carb, 600 holley ).