A gaseous fuel and air mixing venturi device is adapted for installation in a gaseous fuel carburetor and has a venturi body defining annular inner and outer venturi passageways which each have a throat of reduced cross-sectional area and have inlet ends in communication with an air inlet of the carburetor. A fuel inlet is configured for connection to a fuel port of the carburetor, and communicates with a plurality of fuel supply ports in the body communicating radially with the inner and outer venturi passages to mix with the air flowing in the passageways.