Image Number 6 for United States Patent #4302404.
A variable venturi carburetor wherein the suction piston is fully closed with respect to the venturi section at engine stop to elevate the negative pressure at the time of cranking, and is opened to a predetermined degree at the time of idling, thereby obtaining stable driving performance at a suitable air-fuel ratio. The suction piston slides, via a spring, into and out from a suction chamber communicating with a mixing chamber via a negative pressure path and has at its head a metering needle to face a metering jet. The low negative pressure occurring in the mixing chamber along with cranking is reliably applied to the gap between the metering needle and the metering jet which is wider than the gap in their matching state at the time of idling. Hence, the engine starts operating with an over-rich air-fuel ratio. The suction piston is shaped in such manner so as to close a venturi section on the upstream side with respect to the base portion of the metering needle at the time of stop of the engine. A venturi-crossing flange is disposed at a location corresponding to the position in the venturi section of either one or both of the suction piston head and a barrel member of the above-mentioned venturi section. Alternatively, an atmospheric pressure is communicated with the negative pressure to the suction chamber in order to enable driving at a set air-fuel ratio with a suitable lift movement.