Abstract: In a decompression unit including a decompression arm, a decompression unit for internal combustion engine is provided in which the followability in the opening-and-closing operation of the air intake valve or the exhaust valve with respect to the cam at high-revolution speeds is satisfactory without increasing the equivalent inertia weight of the valve driving system, and for lowering the cost. A decompression unit includes a first decompression cam that takes the operating position when the number of revolutions of the engine is not more than the predetermined number of revolution during startup. A second decompression cam is provided that takes the operating position by manual operation. A decompression arm includes first and second abutting portions that abut against first and second decompression cams, respectively.

Abstract: In an internal combustion engine, a pair of suction ports are formed on left and right sides of the rear of the cylinder head with respect to the vehicle body. A pair of exhaust ports is formed on left and right sides on the front side of the cylinder head. Suction poppet valves and exhaust poppet valves for opening and closing the suction ports and the exhaust ports are slidably fitted in guide tubes, respectively. In addition, a valve gear includes a camshaft and a rocker arm pivotally supported on a rocker arm shaft. The rocker arm is formed with a bifurcated end, which crosses an ignition plug on the exhaust poppet valve side. The suction poppet valves are directly driven to open and close by the camshaft.