If it is running too rich, check the top and center castings for warpage. Warpage can cause an internal vacuum leak, causing the power valve to be open all of the time. The Rochester B is infamous for this issue.
And even though you are dealing with the Rochester B, remember that 93 percent (at least with real carburetors
) of all carburetor problems are ignition!!!!! The perfect mixture, combined with a weak ignition will generally run rich and blow black smoke. Items often causing issues are old or incorrect spark plug wires, and old are generic (thus incorrect) coils.
If it is the carburetor, one other possible issue is the fuel valve. The original fuel valve was a smaller orifice than comes in many modern repair kits. This can cause the fuel level in the bowl to be too high (as can too much pressure from the fuel pump).
And if everything else fails, remember the ABC's of carburetion (A)lways (B)uy (C)arter!
Jon.
