While I don't really like to attempt to answer questions on Rochester B's (other than replace them), there are other issues here that I would like to address.
The engine running hot, as posted by Baldy, is more likely to be a timing problem. However, if the hot condition is caused by fuel, it would be from too LITTLE fuel, not too much fuel! The change of state (liquid to gaseous) of gasoline requires external heat (the "heat of vaporization"). Many racers run A/F mixtures richer than the point of maximum power simply to increase cooling efficiency.
Vacuum, signal and airflow are separate entities!
With 2 carburetors running simultaneously, the AIRFLOW through each carburetor will be cut in half. The VACUUM will probably remain about the same at idle; but will probably decrease more at WOT than with the single carburetor.
As airflow is decreased, signal also will be decreased. Signal is dependent on air velocity, and air velocity is dependent on air flow.
Unless someone has an identical setup on an identical engine that has been professionally tuned, jetting should begin with the stock jetting for the carburetors. If one picked the correct size carburetors (THE MOST CRITICAL PART OF TUNING!!!), then the stock jetting will be close enough to do no damage and allow further tuning. Due to the decrease of signal, it is common to need to increase the size of the jets slightly; but this is not always the case!
Where vacuum really enters the calibration picture is the operation of the power system in a vacuum power system carburetor. As the vacuum is likely to be less (although never half) as throttle opening is increased; it is likely that the power circuit will open too quickly, thus creating an overrich condition at part throttle. It is generally necessary to tune the timing of the power circuit to operate with the reduced vacuum. Utilizing carburetors with a mechanical power system makes for a unit which is much easier for the hobbyist to adjust.
Attempting to use carburetors which are not close to the correct size, will normally result in either total frustration; or spending an inordinate amount of time/money trying to compensate for using the wrong size carburetors.
Remember the most expensive carburetor one will ever own, is the carburetor given to one that is incorrect for the application!
Jon.
