Abstract: A split vapor/liquid fuel supply system feeds liquid hydrocarbon fuel to the carburetor or its equivalant of an spark ignition internal combustion engine for ignition with air within a reciprocating piston defined combustion chamber. A partial fuel vaporizer connected intermediate of the fuel tank and the carburetor includes a housing forming underlying liquid fuel chamber, an overlying vapor expansion chamber, and a heat transfer plate separating the same in a sealed housing assembly. Varying degrees of vacuum pressure are applied within said sealed housing chamber causing ambient air to enter the air liquid chamber below the liquid fuel level within the liquid fuel chamber for percolation therethrough, passage through a perforated air distribution plate, and explosion impingement against the perforated heat transfer plate with increased production of vaporization of the liquid fuel due to the lowering of the fuel boiling point as a result of atmospheric pressure with said chamber.