A carburetor which comprises a casing, an air inlet at the upper end thereof and an air-fuel mixture discharge at the lower end thereof, with a venturi throat located downwardly of said inlet, a sonic throat below said venturi throat, and a diffuser section interconnecting the sonic throat and the discharge. Portions of the wall of the sonic throat are provided with a plurality of indentations, projections, or surface irregularities for stabilizing the flow therethrough and conducing to the uniformity of the air-fuel mixture. At least one portion of said sonic throat is movable with respect to the fixed portions of said throat for altering the cross-section thereof responsive to fuel requirements for combustion.