Abstract: A combustible air-liquid fuel mixture having a substantially constant air-to-fuel ratio is produced for delivery to the intake manifold of an engine. Air is passed through a fluid flow device having a variable area throat zone to increase the velocity of the air to sonic, and the area of the throat zone is varied in correlation with operating demands imposed upon the engine for which the mixture is produced. Liquid fuel is metered from a supply into the air stream at or before the throat zone in direct proportion to the cross-sectional area of the throat zone. The pressure of the high velocity air stream is sensed at a point where it bears a predictable relationship to atmospheric pressure, and the rate of fuel delivered into the air stream is adjusted in response to changes in the air pressure sensed so that the air-to-fuel ratio of the mixture is maintained substantially constant.