Abstract: A rotary lawn mower having an air-cooled single cylinder four-cycle internal combustion engine for powering its cutting blade. The engine is air cooled and is disposed so that its exhaust port lies above the intake port and the carburetor is positioned above a raised portion of the scroll part of the lawn mower housing for good cooling of the exhaust port without heating of the carburetor. A valve operating mechanism is disclosed including a pair of push rods that are supported on the exhaust port side of the engine and which are surrounded by a mass of cylinder head and cylinder for assisting in heat dissipation.