Abstract: The present invention relates to a system (1) for supplying an air-fuel mixture to a two-stroke internal combustion engine cylinder (2), comprising a carburettor (3) having a throttle valve (4) and usually a choke valve. The system further has a transfer passage (6) between a crankcase chamber and a combustion chamber of said engine. A supply conduit for additional air (8) is having an air valve (10) for said additional air, and said supply conduit for additional air (8) is being adapted to discharge the additional air into the top portion of said transfer passage (6) during a portion of a cycle of said two-stroke internal combustion engine. Further the air valve (10) is controlled by the throttle valve (4) so as to affecting a air valve opening state when said throttle valve opening state is affected. Said air valve (10) is of a barrel valve type while said throttle valve (4) is of a butterfly valve type.