The carburetor (1) includes a flow duct including rich (60) and lean (50) flow passage in parallel, through which, in use, air flows in a flow direction and which are separated by a substantially planar partition (30), at least one fuel jet 5 communicating with the rich passage (60), the partition (30) including an aperture (40) towards which the fuel jet (5) is directed, and a substantially planar butterfly valve (20) being received in the aperture (40) so as to be pivotable between a first position, in which the flow duct is substantially closed and the aperture (40) is substantially open, and a second position, in which the flow duct is substantially open and the aperture (40) is substantially closed, the upstream half of the aperture (40) being defined by an upstream semi-annular seating ledge (48) affording an upstream seating surface which is engaged by one of the surfaces of the butterfly valve (20) when it is in the second position and a first end surface which extends between the upstream seating surface and that surface of the partition (30) which is directed towards the lean passage (50), the downstream half of the aperture (40) being defined by a down-stream semi-annular seating ledge (49) affording a downstream seating surface which is engaged by the other surface of the butterfly valve (20) when it is in the second position and a second end surface, which extends between the downstream seating surface and that surface of the partition (30) which is directed towards the rich passage.