The invention relates to a valve operating mechanism with roller rocker arm, a 4-cycle engine, and a motorcycle having 4-cycle engine mounted thereon. A valve operating mechanism (17) comprises a cam shaft (18) having valve operating cams (25, 26), first and second rocker shafts (30, 38), a first rocker arm (19) supported by the first rocker shaft (30), and a second rocker arm (20) supported by the second rocker shaft (38). The valve operating cams (25, 26) have cam noses (27b, 28b) projecting from base circles (27a, 28a), respectively, and the first rocker shaft (30) supporting the first rocker arm (19) is positioned more forwardly of the direction of rotation of the cam shaft (18) than a centerline (O1) extending axially of a cylinder (4) through the center of the cam shaft (18). When the roller bearing (34) of the first rocker arm (19) is in contact with the base circle (27a) of the valve operating cam (25), the first rocker shaft (30) is more offset than the center of rotation (X2) of a roller bearing (34) as seen in the direction in which the cam shaft (18) is approached. |