Abstract: A propane gas feed device and system are provided wherein controlled, measured quantities of propane gas are controllably pre-mixed with air and this pre-mixture of air-propane is fed as a carburetor mixture for combustion in the engine. An air cut-off screen is controllably operated to adjust the air-fuel ratio relative to a changeover in the propane and gasoline demands. The propane feed system may be retro-fitted to an automobile employing conventional propane feed and provides for more efficient use of the propane with reduced unburned fraction in the engine exhaust. The propane air-feed device is also useful as wherein propane is the sole fuel.