Vapors normally arising within a standing gasoline tank are captured in a charcoal filled cannister. When the engine is running, air is drawn through the cannister thereby purging gasoline adsorbed on the charcoal within the cannister and delivering the purged gasoline vapors to the carburetor ultimately for combustion within the engine. Interference by the presence of such accumulated gasoline vapors in the carburetor, particularly during warm restarts and idling, can be avoided by preventing the injection of these gasoline vapors into the carburetor at the inappropriate times. Selective withholding of the purged gasoline vapors from a piston type carburetor, having a piston throttle valve disposed into and out of a venturi portion of the carburetor, is accomplished by injecting the gasoline vapors into the venturi portion through a purge port within the carburetor at a point such that the leading edge and outside surface of the piston type throttle valve closes and seals the port when the throttle valve is in a fully closed position. Thus, during idling and warm restarts, when the throttle valve is closed, no evaporated vapors are allowed to fill the venturi portion of the carburetor. Purging of the gasoline adsorbed on the charcoal within the cannister is accelerated by increasing the effective area of the purge port as the throttle valve is opened.