At idle speeds, channel (B) is below the throttle plate (1) and no suction is transmitted to the fuel bowl. This permits the full flow of fuel through the idle system. At approximately 1/4 open throttle, channel (B) is above the throttle plate and a strong suction is transmitted to the fuel bowl through channels (C) and (D) to hold back the fuel in the fuel bowl which results in a lean mixture during part throttle range of operation. The size of the economizer jet (3) controls the suction applied to the fuel bowl. A restriction (E) in the air intake passage fo rbowl ventilation prevents the bowl ventilation from over ruling the suction channels (B) and (C). At wide open throttle this suction is cut off, due to the rotation of the slot (A) in the throttle shaft, to permit the full flow of fuel necesary for full power.