You'll notice the "3.3" correction in my math. This estimates how much greater area is need for less than 7 cylinder fed by the same carburetor(s) in common. This is the maximum correction, it decays with runner volume. The correction for the common CFM formula by cylinders served by a single source (1 or more throats in common):
1 cylinder 6.6
2 cylinder 3.3
3 cylinder 2.2
4 cylinder 1.6
5 cylinder 1.3
6 cylinder 1.1
7 or more cylinders 1
If the carburetors are large enough that the WOT vacuum drops to 1.5" Hg multiple CFM by .707.
If you have paired intakes (even if not siamese), not a common plenum, the 3.3 factor is correct. If on the same plenum, use 1.6.
If this were a parallel twin motorcycle (giant Triumph) it would be 87.5" or 1434cc using a single carburetor. Even the 32mm is much smaller than optimum (some 650cc twins used 2 X 32mm).
Sorry, I can't guess at the Zenith CFM.
