apparatus including a mixing chamber is installed between a standard carburetor base and the intake manifold of a gasoline engine . the mixing chamber includes an air inlet port for admitting a controlled amount of outside air into the mixing chamber and a gasoline vapor port for admitting gasoline vaporized externally of the carburetor into the mixing chamber , a carburetor inlet port for receiving a mixture of air and partially vaporized gasoline from the carburetor throat , and an outlet port through which a mixture of all the gases exhausts into the intake manifold . the carburetor includes a first throttle valve for controlling the air amount and partially vaporized gasoline passing into the mixing chamber . a second throttle valve controls the amount of mixed gas drawn through the mixing chamber outlet port into the intake manifold . a linkage connected to the automobile accelerator cable opens the second throttle in response to initial accelerator displacement . as the engine load increases and the accelerator is depressed further , the second throttle valve opens further and the linkage causes the first throttle valve to partially open , admitting a normal air charge and partially vaporized gasoline into the mixing chamber along with the air and gasoline vapor admitted through the two ports thereof . during cold start , while an automatic choke is closed , the first throttle valve opens . the linkage forces the second throttle valve open , and a rich mixture suitable for cold engine conditions passes through the mixing chamber into the intake manifold .