PURPOSE: To enable to compact an engine by having a link mechanism for connecting an engine speed fluctuation value detecting mechanism and a carburetor on the engine side inserted through under the higher one between two air cylinders offset vertically.

CONSTITUTION: In an engine, the link mechanism 29 of a governor device 30 is provided under the air cylinder 25b which is in the higher position between two offset air cylinders 25a, 25b. As a result, the link mechanism is prevented from protruding in the cross or altitudinal direction of the engine 2. Accordingly, the entire engine becomes compact as well as the vehicle mounted with this engine is prevented from being large sized as the link mechanism is not interfered with other parts.