A spud end fuel metering tube in an air-fuel mixing passage in a propane carburetor for enhancing performance of the system. The fuel supplying spud end is located in a venturi throat within the mixing chamber of the carburetor. The spud end includes linearly aligned fuel supply apertures that face downstream of the airflow therethrough. This combination provides increased power and fuel economy over a wide range of RPMs and demand levels while substantially reducing undesirable pollutant emission levels. The butterfly valve of the carburetor includes apertures formed therein for permitting the engine to idle when the butterfly valve closes eliminating the necessity for other downstream ports.