manifold vaccum for vaccum advance ?'s

When you have a fairly radical cam and less then 12" or so vacuum then i could agree with the ported vacuum as it could make it irratic at idle. But you have a higher vacuum which i figured with that crate engine. At this point, if it just started, you have to look at whats been changed. The carb wasn't touched so i would rule that out for now. The distributor, the wiring for it and the plug wires are all parts that have had attention at this point. That's the first place to look as any of these could be a issue. If you have a spare coil it may not hurt to try it cause it's easy to do.