movable between a closed position wherein the valve 70 abuts a seat 71 defined by that portion of an inner wall of the air horn 38 surrounding the opening 64 and a second position remote from the seat 71 the valve 70 includes an annular stop member 72 and a tapered side wall air metering portion 73 stop member 72 abuts the seat 71 in the closed position of the valve 70 the tapered portion 73 permits increasingly larger quantities of air to enter the chamber portion 39b through the opening 64 as the bypass valve 70 moves in a direction away from the seat 71 a fitting 69 is mounted along the upper portion of the chamber portion 39b the fitting 69 has a flange 74 extending radially inwardly from it which defines a seat for one end of a spring 75 the second end of spring 75 is seated against a stop valve 76 stop valve assembly 76 is attached to the valve stem 65 near the end 67 of the stem 65 valve 76 includes a broad diameter upper part 77 and a narrow diameter part 78 the lower part 78 includes an annular closure member 79 having a tapered side wall 80 extending downwardly from it the end 67 of the valve stem 65 is a dual diameter device that includes a first part 84 and a second part 85 in the embodiment illustrated the part 85 has a smaller diameter than the part 84 the dual diameter construction of the end 67 provides a second metering function during the operation of the auxiliary fuel supply system 100 as later explained the lower end of the channel portion 39b.