PURPOSE: To prevent the decrease in the quantity of intake, by applying the increased pressure in a crankcase to the upstream side of a carburetor in the process of the intake.
CONSTITUTION: A connecting section 13 having an opening 12 is provided on one end of a crankcase 11. A connecting section 14 extends to a part of an expansion chamber 1b which is a portion of an air cleaner 1. A diaphragm 15 is interposed between the connecting sections 13, 14 so that these sections are coupled to each other. Since the pressure in the crankcase 11 rises with the descent of a piston 7 in the process of intake, the diaphragm 15 is expanded toward the expansion chamber 1b so that air in the connecting section 14 is pushed out to the upstream side of a carburetor 4 to heighten the pressure at the side. As a result, the pressure difference between the upstream and downstream sides of the carburetor 4 is augmented to increase the quantity of mixture to be sucked into a cylinder 8.