Crankcase scavenged two-stroke internal combustion engine (1), in which a piston ported air passage is arranged between an air inlet (2) and the upper part of a number of transfer ducts (3, 3′). The air passage is arranged from an air inlet (2) equipped with a restriction valve (4), controlled by at least one engine parameter, for instance the carburetor throttle control. The air inlet extends via at least one connecting duct (6, 6′) to at least one connecting port (8, 8′) in the engine's cylinder wall (12). The connecting port (8, 8′) is arranged so that it in connection with piston positions at the top dead center is connected with flow paths (10, 10′) embodied in the piston (13), which extend to the upper part of a number of transfer ducts (3, 3′). Each flow path through the cylinder and piston is to a great extent arranged in the cylinder's lateral direction, on the one hand in that the connecting port (8, 8′) and adjacent scavenging port (31, 31′) of the cylinder are shifted sideways in relation to each other along the periphery of the cylinder wall (12), and on the other hand in that the transfer ducts (3, 3′) of the cylinder are running essentially in the cylinder's lateral direction away from each transfer port (31, 31′) respectively, i.e. tangentially in relation to the circumference of the cylinder wall (12).