The following is copied from an article on our website, and adds to what Jerry has posted:
"For best results, the very best thing is to do one’s homework BEFORE one purchases anything. There are several major criteria in the selection of one’s puchase, but the first criteria is how the unit will be used. Consider that 50~60 years ago, dual carb manifolds were sold for RACING! Today, while a very small percentage will submit their vehicles to the race track, most are interested in dual carbs for either LOOKS ONLY, or looks with a modest increase in performance. These two uses have vastly different requirements in carburetors.
Typically, the manifold sold in the period would be drilled for two carbs the same size as the original (a 216 Chevrolet would have 2 S. A. E. size 2 carbs, whereas a 235 Chevrolet would be drilled for 2 S.A.E. size 3 carbs). By using this arrangement, the user could order a second original carburetor from the dealer to go with original carb on the engine. At the same time, the user would install a full-race cam, a magneto ignition, headers, high-compression pistons; all to go with a numerical higher rear-end ratio BECAUSE THE ENGINE WOULD HAVE AT BEST A VERY HIGH IDLE, AND VERY LITTLE LOW END TORQUE. This is a good selection for a trailered race car, but a poor selection for the “cruiser” on the street.
Since the manifolds of today are basically copies of the manifolds of yesteryear, generally the street enthusiast will need to modify the manifold for smaller carburetors. Example: for a stock or modestly built Chevrolet 235, the very best carburetors would be those designed for a Chevrolet 216. The manifold would be drilled for size 3 carbs (2 15/16 inch mounting pattern), but the 216 carbs are size 2 (2 11/16 inch mounting pattern). There are three methods of doing the conversion: (A) use size 3 to size 2 adapters (extra cost, and looks “hokey”); (B) slot the holes in the carburetor (no additional cost, but looks even more hokey); or (C) have a good welder fill the original holes in the manifold, and then drill and tap for the correct size (extra cost, but looks much more like a factory installation, and weren’t looks the major criteria?).
If you plan to use on a trailered racecar, use the larger size."
End of quotation.
And a bit more information:
There are three major methods of improving the atomization of fuel:
(1) Increase velocity
(2) Increase the amount of fuel (and watch some of it drip out the tailpipe)
(3) Add heat
A carburetor is calibrated for a specific venturi air velocity. If too large a carburetor is used, then the venturi air velocity will be lower than the design for the calibration. And thus will cause the engine to run lean.
Jon.
