I posted this link about vacuum advance the other day:
http://www.classicchevytrucks.20m.com/vacuum.htm To answer your question directly, there should be zero vaccum advance at heavy or full throttle.
I think we need a better definition of "burble." If you mean that the engine stumbles and misses at part throttle, it could be "lean misfire" caused by the primary jets (or what have you) being too lean. If it just happens when you apply part throttle, then it might be corrected with an accelerator pump adjustment or replacement.
Another thing to check would be your static timing (at idle with the vacuum advance hose disconnected) and then the dynamic timing by running the motor up to around 3000 rpm (all cnetrigugal timing should be in by then.) A dial-back timing light would be helpful with this.
