Blow-by gas processing circuit for internal combustion engine

A blow-by gas processing circuit for an internal combustion engine, in which a carburetor having an intake passageway extending laterally is to be connected to one side surface of an internal combustion engine with an insulator member interposed between the carburetor and the one side surface of the engine, and an air cleaner placed above the carburetor is connected to an upstream end of the carburetor, wherein

• the air cleaner includes one side surface that is formed on a lateral portion of the air cleaner exposed to an outside and extends in an up-and-down direction,
• the blow-by gas processing circuit comprises:
• a blow-by gas lead-out pipe which is configured to rise upward from the internal combustion engine, and run along the one side surface of the air cleaner;
• an upper pipe bending laterally from an upper end of the blow-by gas lead-out pipe, and running along the one side surface of the air cleaner;
• a descending pipe bending downward from an extremity portion of the upper pipe, and running along the one side surface of the air cleaner so as to reach a purification completion chamber of the air cleaner;
• a branch pipe branching off downward from an intermediate portion of the upper pipe, running along the one side surface of the air cleaner, and connected to a blow-by gas outlet port which is opened to an intake passage penetrating the insulator member;
• and
• a control valve placed in an upper portion of the branch pipe, and configured to open in accordance with a rise in boost negative pressure of the internal combustion engine,
• wherein substantial portions of the blow-by gas lead-out pipe, the upper pipe, the descending pipe and the branch pipe are aligned in one direction orthogonal to a running direction of the intake passageway.