 While basic carburetors have only one venturi, many carburetors have more than one venturi, or "barrel".Two barrel and four barrel configurations are commonly used to accommodate the higher air flow rate with large engine displacement.Multi-barrel carburetors can have non-identical primary and secondary barrel(s) of different sizes and calibrated to deliver different air/fuel mixtures; they can be actuated by the linkage or by engine vacuum in "progressive" fashion, so that the secondary barrels do not begin to open until the primaries are almost completely open.