I know this is most likely not the problem but, is it the same 390 the circle track guys have to run to limit horse power? Would those carbs have large air bleeds to help with a rough idle. Maybe it is loading up at idle and when you get into the main circuit with a jet change it is better. My idea would be swap to a carb someone you know has that has worked on other engines. a 1850 600 holley is made for a STOCK engine and will have air bleeds that are small. It may not have as good a throttle response but may fix your idle. What is you list # for your carb? Were are your air screws at when adjusted for the best idle?
