What X89 said.. Also check the advance with a light! while slowly advancing the rpm watch the advance to see when the vac advance and then the Mechanical advance comes in.. then drive it and see where the flat spot is in rpm..
By doing this you should be able to surmise what changes in the mech advance are needed.
If the carb is in good health I doubt the carb is the problem. Flat spots when FLOORING the pecal are from not enough fuel in the pump system this is changable with the linkage on that carb.
I put my dist vac on a constant source and enjoy better ideling. You may try this to see what happens.
Just a guess but I'd betcha your mech advance is coming in a bit late..
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />