To prevent a large quantity of blow-by gas filled in a crank chamber from being instantaneously supplied to an engine via a carburetor at the time of a sudden speed reduction operation of the engine and to prevent a failure or stop of the engine due to a rapid increase in fuel quantity.

This mechanism is provided with a cylindrical valve chest 57 which crosses an intake passage 16 penetrating in a carburetor main body 15 and extends in the vertical direction. In the valve chest 57, a throttle valve 47 having a throttle hole 47b is rotatably and elevatably fitted and inserted. A fuel nozzle 56 communicated with a constant pressure fuel chamber 25 supported on a bottom wall of the valve chest 57 is projected to the throttle hole 47b and a needle 55 supported on the throttle valve 47 is fitted and inserted into the fuel nozzle 56 and opening of a fuel nozzle 56a is regulated. An end part of a passage 71 extending from bottom space 57a of the valve chest 57 to an upstream side end part of the carburetor main body 15 is coupled with a connection pipe 72. An end part of a pipe 73 extending from a crank chamber of the engine via a reed valve is connected with the connection pipe 72.