Try re-jetting your Rochester Bs. Here is a resource for you:

http://quadrajetparts.com/rochester-1bbl-parts-jets-c-241_270.html

I am running a 50 jet instead of the original 56 to 58 that came standard on my Rochester B. I am getting around 20-25 MPG and have a dark gray tailpipe.

Walt Williams
La Verkin, UT