I just had the same problem with my C, i bought a carb rebuild kit from a friend and the gasket between the bottom/top half was wrong, i guess all of the gaskets are or something, there's a hole that you need to make to let the air above the gas in the float side let the air back into the intake side, it's just a little tiny hole, if you look at your old gasket you might be able to see it. this might not be your problem, but after having mine apart at least 4 times before i asked my friend about it, i figure it's an easy task just to look at it....
just an idea, good luck
Johnny