Not saying this is your problem, but certainly possible:
One of the design issues with the Rochester B is casting warpage due to too few screws attaching the air horn to the bowl. There is an internal vacuum passage to operate the power valve. When sufficient warpage occurs between the air horn and the bowl, the passage is sufficiently exposed to reduce the existing vacuum in the passage below the threshold required to keep the power valve closed. I have yet to see a Rochester B that this phenomena did not eventually occur.
When it happens, gas mileage is significantly reduced. When It first occurs, black smoke does not appear at idle in the driveway, as there is sufficient vacuum at idle to maintain the threshold.
If this is the issue, there are two solutions: (1) straighten the castings using a special jig which can be fabricated, and heat; (2) simply replace the B's with something without this issue.
Generally, the castings last about 5 years after straightening before the issue reoccurs, but it eventually will reoccur.
And the vacuum advance mentioned in the above post is certainly a possibility.
Jon.
