Abstract: An air flow directing ramp in the air intake passage of a horizontal updraft carburetor defines an air flow passage of reduced cross sectional area which, among other functions, establishes an increased air flow velocity during starting operations and directs this flow to the main nozzle of the carburetor. The main nozzle is provided with an outwardly flared discharge extension having restricted air passages which substantially improve the mixing action. The pivotally mounted choke plate, located upstream of the air intake passage from the velocity ramp is formed with a restricted opening constituting an air inlet to the air flow passage of the ramp when the choke is closed, while an upwardly curved lip portion of the choke plate cooperates with the upwardly curved velocity ramp when the choke is opened to provide a smooth deflection of intake air from the horizontal intake passage to the vertical mixing chamber.