Might also be a loose venturi in the carb - that will cause all sorts of magical problems. A local guy told me to take the carb off, remove the gasket between the upper and lower body halves, then set the two halves back together. If the two halves fit together flush then the venturi's too loose. There should be a slight gap between the body halves when you so this - the tabs on the venturi hold them slightly apart unless the venturi tabs are worn.
My B was like that. I took a small piece of 17 gauge aluminum electric fence wire, made a circle, and beat it flat to make a thin spacer. I then put that under the venturi tabs. Helped out a *lot* (it's not perfect yet but there was a large difference).
