A two-stroke internal combustion engine comprising: - a base, - a head fixed to the base and in which a cylindrical cavity is formed, - a piston slidably received in the cylindrical cavity, so as to define a combustion chamber and a pumping chamber, and movable in the cylindrical cavity between a bottom dead center, in which the volume of the combustion chamber is maximum and the volume of the pumping chamber is minimum, and a top dead center, in which the volume of the combustion chamber is minimum and the volume of the pumping chamber is maximum, - a transfer duct provided with an inlet mouth adapted to be placed in fluid communication with the pumping chamber, and with an outlet mouth adapted to be placed in fluid communication with the combustion chamber, - an exhaust duct provided with an inlet mouth adapted to be placed in fluid communication with the combustion chamber, - a crank shaft at least partially housed in the pumping chamber, - a connecting rod adapted to connect the piston to said crank shaft, wherein the two-stroke internal combustion engine implements, by means of the movement of the piston between the top dead center and the bottom dead center, an operating step during which the inlet mouth of the exhaust duct and the outlet mouth of the transfer duct are simultaneously in fluid communication with the combustion chamber, said two-stroke internal combustion engine comprising a movable partition housed inside the pumping chamber and operatively connected to the crank shaft to occlude the inlet mouth of the transfer duct during a first portion of said operating step, and to put in fluid communication the inlet mouth of the transfer duct with the pumping chamber during a second portion of the operating step, wherein the end of the first portion of the operating step is between 30° of rotation of the crank shaft before the bottom dead center and 30° of rotation of the crank shaft after the bottom dead center, wherein the movable partition comprises: - a disc-shaped body, which is rigidly integral in rotation to the crank shaft and is positioned so as to occlude the inlet mouth of the transfer duct at least during the first portion of the operating step, and - a through slot formed in the disc-shaped body and positioned in such a way as to place in fluid communication the inlet mouth of the transfer duct with the pumping chamber during the second portion of the operating step. said two-stroke internal combustion engine being characterized in that the movable partition comprises a further through slot made in the disc-shaped body, in a portion of the disc-shaped body itself proximal to the crank shaft. - a base, - a head fixed to the base and in which a cylindrical cavity is formed, - a piston slidably received in the cylindrical cavity, so as to define a combustion chamber and a pumping chamber, and movable in the cylindrical cavity between a bottom dead center, in which the volume of the combustion chamber is maximum and the volume of the pumping chamber is minimum, and a top dead center, in which the volume of the combustion chamber is minimum and the volume of the pumping chamber is maximum, - a transfer duct provided with an inlet mouth adapted to be placed in fluid communication with the pumping chamber, and with an outlet mouth adapted to be placed in fluid communication with the combustion chamber, - an exhaust duct provided with an inlet mouth adapted to be placed in fluid communication with the combustion chamber, - a crank shaft at least partially housed in the pumping chamber, - a connecting rod adapted to connect the piston to said crank shaft, wherein the two-stroke internal combustion engine implements, by means of the movement of the piston between the top dead center and the bottom dead center, an operating step during which the inlet mouth of the exhaust duct and the outlet mouth of the transfer duct are simultaneously in fluid communication with the combustion chamber, - a disc-shaped body, which is rigidly integral in rotation to the crank shaft and is positioned so as to occlude the inlet mouth of the transfer duct at least during the first portion of the operating step, and - a through slot formed in the disc-shaped body and positioned in such a way as to place in fluid communication the inlet mouth of the transfer duct with the pumping chamber during the second portion of the operating step.