Is there a black wire with a white stripe on the side of the carb that is/isn't hooked up? It's for the fuel cut solenoid. This bad boy needs to be hooked up or it'll shut off the internal air metering in the carb and do things like randomly kill the engine (or perhaps in this case mess up fuel delivery enough to cause it to run badly) It is powered by the ignition circuit - no power, no fuel.