RE: technical advice requested
One more thing.
If it had been backfiring and you replaced the vacuum advance because it was bad, it's possible you developed in intake manifold gasket leak under the carb or by the heads.
With the engine at idle, spray WD-40 along the side intake gaskets at the heads and under the carb.
If it's sucking air there your RPM's will increase with the richer mixture.
DO NOT USE GAS!! One backfire and poof no more bird.
If you fine a leak try tightening everything to the high end of the torque spec's for the bolts and nuts before replacing the gaskets.
LET EVERYTHING EVAPORATE BEFORE YOU TRY THE ACCELLERATOR AGAIN OR YOU MAY GET A FIRE.