That's my experience with the B's that I have played with, too.
The differance between a 49 and a 52 jet is too small for you to notice that kind of a drivability problem. Also, under acceleration, the power circuit is much more of a dominant fuel provider. The main jet really only affects cruise on the "b", really just plug fouling and fuel economy.
I'll bet you either need a new float (they get leaky, even the brass ones), or a level adjustment. I fought my B until I got that problem fixed, and its been reliable for 4 years since- haven't had to touch it.
I like to chamfer the edge of the fuel valve that comes with the kit by placing one of the old steel balls from the rebuild kit in it and giving it a light whack with a punch and hammer. This gives the viton seat of the needle a bit more surface area to seal and keeps it from sticking. It means you have to adjust the float level by checking the amount of gasoline in the bowl instead of measuring it, but it seems to work well.
I learned that trick from messing with the fussy Keihin carbs on my motorcycle that have floats that are too small for the design.
