The present invention relates to a device arranged to enrich or lean the mixture from the low speed and progression circuit of a carburetor, in order to adapt the air-fuel ratio of the mixture to the various conditions of operation which concern this circuit exclusively.  More particularly, the invention relates to a device which is part of the low speed circuit and affects the quantity of emulsion air, in order to control the air-fuel ratio of the mixture delivered with small openings of the throttle valve. 
Devices of this type are known. 
To correct the air-fuel ratio of the mixture delivered by the low speed circuit of a carburetor, in order to keep it adapted to the various operation requirements of the engine, it has been proposed to vary the gasoline metering port in a continuous or discrete manner, subjecting such variation to the manifold vacuum in the exhaust conduits downstream of the throttle valve. 
When choosing to carry out the correction of the air-fuel ratio by acting on the gasoline, considerable expenses have to be met for machining the metering parts which have to be of very high precision.  In fact, an error in the dimensions of such a jet gives rise to a difference in the air-fuel ratio of mixture which difference is up to 15 times greater than the difference in the same air-fuel ratio which derives from an error of the same order in the dimensions of a jet which meters the air.