VALVE SYSTEM OF FOUR-CYCLE ENGINE

PURPOSE:To improve an engine output over a wide engine speed area by forming a large diameter part on a rocker shaft, fitting medium and high speed rocker arms to the large diameter part, and fitting a low speed rocker arm to the rocker shaft between the medium and high speed rocker arms. CONSTITUTION:When an engine is under a low speed area, for example, a rocker shaft 11 is rotated in a direction of an arrow 0 and thick parts of an eccentric bush 12 are moved obliquely frontward by the operation of a hydraulic cylinder 15 through the control of an ignitor. Thus, cam floor faces 8c, 9c of medium and high speed rocker arms 8, 9 are moved relatively downward in respect to a cam floor face 7c of a low speed rocker arm 7. The low speed rocker arm 7 is, at this time, pressed up by a valve spring, and its cam floor face 7c is abutted against a peripheral face of a low speed cam 3. When a cam shaft 6 is rotated, accordingly, valves 1, 2 are moved vertically based on a lifting characteristic A of the low speed cam 3.