Rotary throttle valve type carburetor

A valve opening mechanism for a rotary throttle valve carburetor having a main body with an air intake passage, a cylindrical throttle valve installed in said main body crosswise in the air intake passage, said throttle valve having a throttle orifice and a metering needle, a push spring which drives said throttle valve toward an idle position, a fuel nozzle installed on a central axial line of said throttle valve, said fuel nozzle opening into said throttle orifice, and receiving said metering needle, a constant-fuel chamber connected to said fuel nozzle, a valve shaft protruding from said throttle valve outside of said main body, and a lever mounted on a tip end of said valve shaft, and wherein the air flow rate and fuel flow rate are controlled by turning said lever in accordance with the operation of the accelerator to move said throttle valve along its central axial line while rotating said throttle valve,

• wherein said valve opening mechanism comprises
• (a) a cam part that causes the aforementioned lever to move from the idle position to a position which increases the flow rate of air and fuel, and
• (b) a return spring,
• said cam part being constructed to be manually moved from an inoperative position to an operative position where said cam part engages said lever in the idle position and places said throttle valve in an "increase" position, said cam part being held in the operative position by the spring force of said push spring, and said cam part being returned to the inoperative position by said return spring when the engagement of said lever is released.