225 Slant needs a good idle. what rpms are you spinning?

Centrifugal advance is just that....
Think about the last time you rode a tilt a whirl/ didn't it seem you were "smashed" into the back wall of the car the faster it spun? The faster the engine turns the farther the weights want to extend from center til they "max out" in the slots, if everything moves freely the springs control how fast they extended out.
The weights could be "stuck" at some point extended out from center, too.

It definitely sounds vacuum leak related the more I read into your posts.
whether it is at the manifold/head interface or within the carb but I bring up timing because the farther it is advanced the faster it will idle, everything else equal.