PURPOSE: To enable output of an engine and to make it compact by arranging a reed valve which is interposed between a crank case and a carburetor, in front of an up cylinder block which projects from the front part of the crank case to the upper part.

CONSTITUTION: An engine E is provided with an up cylinder block 163 which is mounted on a car body frame at a position between front and rear wheels and projects from the front part of a crank case 11e to the upper part. Inside the crank case 11e, at least there are provided a carburetor which is communicated through a lead valve 29 and a transmission M which is contiguously arranged at the rear of the crank case 11e. By this constitution, the read valve 29 is arranged in front of the up cylinder block 163. The read valve 29 is arranged so as to approach a crank shaft 1 of the engine E without being interfered with the transmission M. Consequently, the effective area of the crank case 11e is compacted, and thereby high output and comactness are attained.