PURPOSE: To improve the output of the engine by communicating a discharge port of a duct having an inlet port opened toward the forward part of a motor cycle body to an inlet chamber, and providing a baffle plate partitioning the chamber to make a front and a rear sections.

CONSTITUTION: The fresh air which has entered a front chamber 20B enters the front part side of a carburetor base 13 while being relatively largely bent in course so as to proceed along the inner side surface of a baffle plate 23, and further enters an inlet passage 51 of a front carburetor 14 to make a mixture, and is supplied to an inlet passage 55 of a front cylinder 17. Thus, approximately same volume of the fresh air is supplied to the front carburetor 14 and a rear carburetor 15, and the respective flow-in volumes between the front and rear carburetors 14, 15 are approximated. The adjusted volume for the respective main jet of the front and rear carburetors 14, 15 can be approximated, and as a result, the output can be easily improved.