Abstract: A system which can be retrofit into an existing conventional gasoline powered vehicle for enabling the vehicle to operate on either gasoline or liquefied propane fuel. The system includes a mixer in the form of an adapter to fit on the top of an existing carburetor. The mixer has a unique spring balanced metering device which controls flow of gaseous propane to the carburetor in proportion to airflow through the carburetor. The mixer is connected to a regulator assembly which receives liquid propane in a first chamber, heats the liquid propane to form a vapor, and feeds the vapor through an idle valve to control idling of the engine. The vapor is also passed to a second chamber of the regulator assembly in response to demand from the metering device which is sensed by a diaphragm actuated gas flow valve. From the second chamber, the gaseous propane is fed to a high speed inlet of the mixer.