fuel mixture is conducted by t passage 36 through inlet 37 into mixing chamber 38 forming a common homogenous air fuel mixture the resultant primary air fuel mixture reverses direction in mixing chamber 38 and passes upwardly through outlets 42 and 43 into intake runners 32 and 33 each of intake runners 32 and 33 extends fore and aft and is adapted to deliver an air fuel mixture to a different one half of the engine cylinders during periods of high engine load such as occur during rapid acceleration secondary throttle valves 49 and 50 open causing a secondary air fuel mixture to be inducted through secondary barrels 45 and 46 through openings 47 and 48 into intake runners 32 and 33 this secondary air fuel mixture enters intake runners 32 and 33 at a location directly opposite the location at which the primary air fuel mixture enters through outlets 42 and 43 this causes the primary and secondary air fuel mixtures to impinge in intake runners 32 and 33 which further improves air fuel mixing in addition because of the location of openings 47 and 48 at substantially the same longitudinal location in runners 32 and 33 as outlets 42 and 43 any air leaking by secondary throttle valves 47 and 48 during idle or other periods when throttle valves 47 and 48 are closed will dilute the primary air fuel mixture equally in both directions fore and aft in intake runners 32 and 33 eliminating any imbalance of the air fuel ratio other preferred embodiments in which hot engine exhaust gas is used to heat the mixing chamber are described in my application ser no 498 960 filed aug 20 1974 which.