PURPOSE: To atomize the fuel and provide the stable ratio of mixture by moving a partition board fixed to a piston valve in a manifold in an interlocking manner with the throttle opening (piston valve) of a carbureter to make the manifold diameter variable.

CONSTITUTION: When the opening of a piston valve 3 of a carbureter 4 is medium to low, the flow speed is rapidly reduced by the rapid increase of the diameter immediately after the piston valve 3, and the mixture is separated into the air and the fuel. Thus, the concentration of the mixture is changed whenver the throttle is opened/closed. The port of the manifold is made square, and a movable plate 2 to change the inner diameter is fitted therein. This movable plate is integratedly fixed to the piston valve 3, or provided so as to be moved through a throttle cable, etc. This constitution allows the manifold diameter variable by the moving plate 2 in an interlocking manner with the movement of the piston valve 3 according to the running condition of the engine, and realizes the atomization of the fuel.