PURPOSE: To substantially improve maintainability by arranging a downdraft carburetor in a manner wherein its float chamber faces to the front of a vehicle.

CONSTITUTION: A downdraft carburetor 19 is positioned in the front of a fuel tank 18 and mounted to a cylinder head 10 in its back surface wall in a condition that the carburetor is tilted to the front from a perpendicular position in a manner wherein its float chamber 24 faces to the front of a vehicle. Since the float chamber 24 faces to the front of the vehicle, a main jet or the like, when it is replaced, of the carburetor 19 can be replaced by removing a cover from the float chamber 24 without removing the carburetor 19 substantially improving maintainability. While because of no necessity for spreading a clearance between the carburetor 19 and the fuel tank 18, its capacity can be sufficiently ensured.