A novel flow control device for metering a preferably gaseous fuel to an internal combustion engine is disclosed.The innovative flow control device maintains a predetermined air-fuel ratio, while automatically compensating for variations in the energy content of the fuel.A rotary-flap airflow meter directly drives a preferably butterfly-type main valve, the latter metering a main fuel dose, calibrated to provide an air-fuel ratio slightly leaner than desired, over the entire engine operating domain.A rotary, or linear, trim valve meters a trim fuel dose, used to additively adjust the mixture to the desired quality.A drive member, rigidly joined to the rotary air flap, drives said trim valve, through a variable-ratio lever mechanism. The lever fulcrum is linearly moveable, independently of cam position, thereby altering the lever ratio and subsequently, the corrective fuel dose, over a substantially wide flow range.