| 摘要 |
PURPOSE:To improve a starting property by slightly lifting up an exhaust valve by means of a pressure reducing cam at the time of compression stroke on starting an engine, and allowing part of compressing pressure to escape and reducing an inputted starting torque. CONSTITUTION:When a driving timing sprocket 24 and a main cam shaft 26 are not rotated or rotated at less than a defined speed, a rocker arm 50 which is energized clockwise by means of an extension coil spring is in a prescribed condition and a pressure reducing cam 42 is projected out of the reference face of an exhaust valve lifting cam 30. That is, since a centrifugal force which acts on the weight of a centrifugal type governor 48 which is rotated in an integrated form with the sprocket 24 is smaller than the tensile force of the spring, the pressure reducing cam 42 operates a rocker arm 35 at the time of compression stroke, to slightly lift up an exhaust valve. An a result, part of compressing pressure is allowed to escape reducing an inputted starting torque, thereby, improving a starting property.
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