Did you replace the manifold to carb gasket? The Rochester has a vacuum passage on the base that goes to both the power piston and the vacuum advance. If you use the wrong base gasket (it has notches) or are missing the heat insulator spacer (it has notches too), you cover up the vacuum passage in the rochester and get exactly the behaviour you describe. It does not affect the idle mixture/vacuum, only light throttle/cruising mixture.
