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 inlet is equipped with a restriction valve (4), controlled by at least one engine parameter, for instance the carburettor throttle control. The air inlet extends via at least one connecting duct (6, 6′) to at least one connecting port (7, 7′) in the engine's cylinder wall (12). The connecting port (7, 7′) is arranged so that it in connection with piston positions at the top dead center is connected with flow paths (9, 9′) embodied in the piston (13), which extend to the upper part of a number of transfer ducts (3, 3′), and the flow paths in the piston are so arranged that the recess (10, 10′; 11, 11′) in the piston that meets the respective transfer duct's port (31, 31′) is so arranged that the air supply is given an essentially equally long or longer period, counted as crank angle or time, in relation to the fuel and air mixture inlet period.