Carburettor

Abstract

A carburettor for a two stroke engine includes a flow duct including rich and lean flow passages () in parallel, through which, in use, air flows in a flow direction and which are separated by substantially planar partition (). At least one fuel jet () communicates with the rich passage () and the partition includes an aperture () towards which the fuel jet is directed. A substantially planar butterfly valve () is received in the aperture so as to be pivotable between a first position, in which the flow duct is substantially closed and the aperture is substantially open, and a second position, in which the flow duct is substantially open and the aperture is substantially closed. The upstream half of the aperture () is defined by upstream semi-annular seating ledge () affording an upstream seating surface (), which is engaged by one of the surfaces of the butterfly valve (), 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 which is directed towards the lean passage. The downstream half of the aperture () is defined by a downstream semi annular seating ledge () affording a downstream seating surface (), which is engaged by the upper surface of the butterfly valve, 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 which is directed towards the rich passage. At least one of the upstream semi annular seating ledge, the downstream semi-annular seating and the valve are so shaped that, in use, a pressure differential is created between the rich and lean passages at the upstream and/or downstream edges of the valve, the pressure in the lean passage being higher than that in the rich passage.