Abstract: An assemblage is provided for effecting thorough mixing of two different air and fuel charges such as a first charge of air and gasoline vapors and a second charge of air and alcohol vapors. The assemblage is constructed in the form of a spacer block assembly to be interposed between a multibarrel carburetor mounting base and the carburetor mounting boss of an associated combustion engine intake manifold. The assemblage is further to be used in conjunction with a multibarrel carburetor modified to form a first air and fuel charge in one of the barrels thereof and a second air and fuel charge in a second barrel thereof. The spacer block assembly defines at least one pair of passages extending therethrough including corresponding inlet ends for registry with the outlet ends of the carburetor barrels and outlet ends for registry with the inlet ends of the corresponding air and fuel passages of the associated intake manifold.