Crankcase scavenged two-stroke internal combustion engine, in which at least one piston ported air passage, with length Lai, is arranged between an air inlet (2) and each scavenging port (31, 31′) of a number of transfer ducts (3, 3′), with length Ls, from the scavenging port to the crankcase. 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 of the piston is arranged so that the air supply is given an essentially equally long period, counted as crank angle or time, as the engine's inlet (22-25), and the length of the inlet into which fuel is added, Li, is greater than 0.6 times the total length of the piston ported air passage Lai and the length of the transfer duct Ls, i.e. 0.6×(Lai+Ls) but smaller than 1.4 times the same length, i.e. 1.4×(Lai+Ls).