Abstract: An inner air vent system for internal combustion engines is disclosed which prevents accidental flush of fuel from the float chamber into the venturi tube of the carburetor as when the vehicle is running on a steep slope or extremely rough surface. The inner air vent system comprises an air passage axially defined in the mounting bosses of the air cleaner, air intake pipe and carburetor, with the air cleaner being located above the carburetor. The air passage has a lower end thereof opened into the air gap inside the float chamber. With this arrangement, tilt of the vehicle even into an extreme position is unlikely to permit the flow of fuel from the float chamber into the carburetor venturi tube, because of the air passage having its upper opening held high enough above the fuel level in the float chamber. Furthermore, the inner air vent system can be constructed without the addition of extra joints and piping work, there is no significant increase in assembling cost.

Abstract: A lubricating system for an internal combustion engine is disclosed. This system includes a pair of passages formed between a crank chamber for reserving oil and an air-oil separating chamber accommodating a mechanism for actuating overhead valves. The pair of passages are constituted by a breather passage formed in the upper portion of a cylinder and the oil-returning passage formed in the lower portion of the cylinder. The breather passage has a valve for allowing a fluid to pass solely toward the air-oil separating chamber and a large oil-circulating resistance is imparted to the fluid flowing toward the air-oil separating chamber through the oil-returning passage, thereby circulating an oil mist in one direction only by using the reciprocal motion of each piston. Even when the engine is inclined, oil is prevented from flowing back from the crank chamber to the separating chamber through the oil-returning passage.