A carburettor includes a flow duct (4) including rich (8) and lean (10) passages in parallel, through which, in use, air flows and which are separated by a substantially planar partition (6). At least one fuel jet (12, 14, 16) communicates with the rich passage (8). The partition (6) includes a circular aperture (20). A butterfly valve (24) is mounted on a pivotal shaft (22) to be pivotable between a closed position, in which the flow duct (4) is substantially closed, and an open position, in which the flow duct is substantially open. The pivotal shaft (22) extends perpendicular to the plane of the partition (6). The pivotal shaft (22) carries a circular obturator disc (26), whose diameter is substantially equal to that of the aperture (20) and which is received in the aperture (20) and substantially closes the aperture (20) at all positions of the butterfly valve (24).