As a general rule (and remember, "general" rules were made to be broken by us privates
) the number of "barrels" should be an even divisor into the number of cylinders.
Two two-barrels on a four or eight (or even twelve) cylinder.
Two one-barrels or three one-barrels or three two-barrels on a six cylinder.
Of course, it is also VERY necessary to size the carbs of whatever configuration for the engine displacement and RPM.
Jon.
