timed vacuum vs manifold vacuum

Oldmanrick is right. At high rpm or full throttle run there is no vacuum and therefor no added advance from the vacuum canister. It doesn't matter whether it is plugged into the ported or full manifold side of the carb.

The only difference between the two is that the manifold side adds timing at idle where the ported side only starts adding timing once the throttle blades open. So the breaking up on high RPM must be coming from something else.

In my opinion, if you are running a street car using vacuum advance is a must and the ported side is the way to go. The only benefit of running the vacuum advance on the full manifold side of the carb is if you are running a high-duration cam.