I had one like that I rebuilt it and put it on the car and it was perfect, the next day it ran just like what your describing. After screwing around with everything I found where a dirt dobber had started building a nest on top of the air bleads, cleaned them out with a pin and it has been running great for 2.5 years. Just something to look at.

Did the motor run fine previously with a different carb?

I am not causing global warming, I am just trying to hold off a impending Ice Age!