Fuel Metering Systems On an engine equipped with a pressure-type carburetor, fuel supply in the idling range is ensured by the inclusion in the carburetor of a. a spring in the unmetered fuel chamber to supplement the action of normal metering forces. b. an idle metering jet that bypasses the carburetor in the idle range. c. a separate boost venturi that is sensitive to the reduced airflow at start and idle speeds.