These carbs have a habit of getting pretty dirty internally. You might start with cleaning the circuits paying special attention to the pilot jet and pilot air jet as this is the circuit that controls the mixture at idle and to 1/4 throttle. I also found quite a bit of debris in the accelerator pump bottom plate. Dirty usually enters here at the tiny rubber sleeve that the pump rod slides through. While you are at it, I'd check the valve lash. Since valves have a tendency to get tighter after a while. you may have a valve or two that won't quite close completely.