An improved overhead valve internal combustion engine is disclosed in which an intake port defined in a cyclinder head for communication at its outlet side opening with a combustion chamber opens at its inlet end in a first side of the cylinder head for connection to a carburetor. An exhaust port also formed in the cylinder head for communication at its inlet side opening with the combustion chamber opens at its outlet end in a second side of the cylinder head that extends substantially perpendicular with the plane of the first side for connection through an exhaust duct to a muffler that is located on the opposite side of the cylinder head to the carburetor. This arrangement can enhance cooling of the cylinder and cylinder heads of an engine and miniturize engine overall size, without shortening the length of the exhaust duct connected between a muffler and exhaust port.