A carburetor for an internal combustion engine utilizes a rotatable air motor to achieve intimate mixing of fuel and air and hence provide a combustible mixture of enhanced burning qualities to the engine. The air motor utilizes a hollow, rotatable hub with fuel jets to deliver the fuel and a plurality of vanes to atomize the same. The air motor is driven by the flow of air through the air horn except when airflow is minimal (closed or nearly closed throttle positions). During this minimal airflow, the engine creates a suction head which is utilized to cause air to flow over a turbine that is coupled with the air motor, thereby rotating the turbine to, in turn, drive the motor. During deceleration, fuel is not required to be supplied to the engine and hence mechanism is provided to block the flow of fuel to the engine in response to closing of the engine throttle valve. The mechanism is also operable by the decrease in pressure caused by the suction head created when the engine descends a long steep grade and hence will continue to block the flow of fuel during such a descent as long as the throttle valve remains closed. Fuel is delivered to the air motor as a result of a suction conduit which has an outlet into the interior of the air horn. Thus, during the minimal airflow this fuel is atomized effectively by the rotation of the air motor and when fuel is not needed, the flow is completely shut off. When it is desirable to increase the quantity of fuel delivered to the air motor to meet the power requirements of the engine, one or more venturi boost rings are employed in the vicinity of the suction conduit outlet to increase the suction and hence the quantity of fuel delivered to the air motor.