sstock mentioned checking for the correct flange gasket. I had the wrong gasket installed which blocked the carb's vacuum port and the engine exhibited the same symptoms you describe. In my case, there is a Bakelite* insulator between the manifold and the carb. There is no gasket used between the carb and the insulator - not all carb kits tell you this.
* I believe it is a Bakelite (phenolic) material
