Cooling system for four-stroke cycle internal combustion engine

A cooling system for a four-stroke cycle internal combustion engine including an oil reservoir chamber for accommodating a lubricating oil which is atomized to generate an oil mist, a cylinder block having an accommodating space extending vertically along the side thereof, a combustion chamber provided in the cylinder block, a valve chamber disposed above the combustion chamber and being lubricated by the oil mist, a spark plug, a crankshaft, a camshaft in the valve chamber, a transmission mechanism for transmitting rotational movements of the crankshaft to the camshaft and accommodated in the accommodating space, a blower fan which is rotatably driven by the crankshaft to send air upward through the accommodating space, and intake and exhaust ports formed in the cylinder block and each having a horizontal portion and a vertical portion that extends laterally in respective opposite directions, said cooling system comprising:

• a cooling air channel extending laterally between the combustion chamber and the valve chamber and through which air from the accommodating space flows, said cooling air channel having a primary air channel formed between the vertical portions of the respective intake and exhaust ports and extending toward the spark plug, and an air guide channel having a pair of air inlets opposed to one another and for guiding the air from said pair of air inlets to an upstream inlet of said primary air channel, said air guide channel being defined by a wall extending laterally along the horizontal portions of the respective intake and exhaust ports and across said upstream inlet of said primary air channel, said wall extending vertically in the accommodating space and including an oil mist passage communicated with the oil reservoir chamber and extending vertically therein to supply the oil mist from the oil reservoir chamber to the valve chamber.