| 摘要 |
PURPOSE:To enable forced air-cooling of an engine and to permit provision of excellent mounting and demounting ability of an ignition plug, by a method wherein a cam shaft is projected from the side opposite to a rotary driving mechanism, and a fan for forced air-cooling is secured to an projection end, and an ignition plug is positioned facing the fan. CONSTITUTION:A single overhead cam shaft 60 is held to a cylinder head 36 between a suction valve 56, inclined backward, and an exhaust valve 58, inclined forward. Rotation of a crank shaft 38 is transmitted to a cam shaft 60 through a timing chain. In which case, the cam shaft 60 is projected in a direction extending opposite to that of a timing chain, and an axial-flow fan 72 for forced air-cooling is secured to its projection end. An ignition plug 74 is disposed obliquely in a manner to be inclined backward based on a plane C including an ignition part A and a central shaft B of a cam shaft 60, and is positioned facing the fan 72. This enables an engine 30 to be forcibly air-cooled without an increase in the size of the engine 30, and enables improvement of mounting and demounting ability of the ignition plug 74. |
