The probelm appears to also be a catch-22. I'm fighting this exact problem on my 455 GTO.
Let me see if you have what I have:
With the vacuum line to the distributor diconnected, and the port on the carb plugged, the idle is low (like 600 RPM) and timing is set to 9 degrees BTDC. Plug in the vacuum advance, and it idles just like you set it. Blip the throttle, it won't come back to idle anywhere below 1200 RPM?
Is this what's happening to yours? If so, then there might be too much vacuum advance from the vacuum canister.
I have not tried to yet (lack of time and lack of sleep causing clouded judgement), but I was advised to limit the amount of vacuum advace and increase the amount of centrifugal advance. I am running a Mallory Unilite distributor, where both of these are easy to adjust. If you are running a stock distributor, I may not be all that much help in telling you HOW to do it, just that this might be the way to go.
I'll let you know if I solve my problem with this approach after the weekend.
