Abstract: A carburetor-fuel preconditioner for internal combustion engines comprising a bubble chamber for holding fuel at a preselected level and for bubbling air therethrough; a multiplicity of catalytic beads located within the bubble chamber in contact with gasoline fuel for producing an air-fuel vapor mixture and for the catalytic cracking of long molecular hydrocarbon chains; and at least one air-fuel vapor mixing conduit located between the bubble chamber and the intake manifold of the engine to provide proper air-fuel ratio to the vapor entering the engine cylinders. A screen may be provided in the bubble chamber to prevent dislocation of the catalytic beads. Each mixing conduit is provided with a plurality of peripherally spaced baffles for creating an air-fuel vortex for superior mixing.