An internal combustion engine (1, 1B, 1C, 1D) for a motor vehicle having a ridable seat, wherein said engine (1,1B) comprises a drive shaft, a first camshaft which controls a plurality of suction valves and a second camshaft which controls a plurality of relief valves, wherein said engine (1, 1B, 1C, 1D) comprises at least a first centrifugal device (2, 2B) for varying the timing of the valves (110, 220), of one of said plurality of valves, with respect to said drive shaft, wherein said first device (2, 2B) comprises: - a driving disc (11, 11B) mounted idle on one of said camshafts (10, 20) which controls said one of said plurality of valves, said driving disc (11, 11B) rotating about the rotation axis (101, 102) of said one of said camshafts; - at least one driven disc (12, 12B) which is integral with said one of said camshafts (10, 20); - drive elements for transmitting the motion between said driving disc (11, 11B) and said driven disc (12, 12B), wherein said discs (11-12, 11B-12B) and said drive elements are configured so as to cause a relative rotation of said driven disc (12, 12B) with respect to said driving disc (11, 11B) when the rotation speed of said discs (11-12, 11B-12B) exceeds a predetermined threshold, - a distribution system which mechanically connects said drive shaft with said driving disc (11, 11B) so as to cause the rotation thereof; said engine comprising a first gear which is integral with said driving disc (11, 11B) and a second gear mounted on the other of said camshafts (10, 20) so that the rotation of said second gear directly or indirectly causes the rotation of said other of said camshafts (10, 20), wherein said second gear directly meshes with said first gear so that the rotation of said driving disc (11, 11B) causes the rotation of said other of said camshafts (10, 20) which controls the other of said plurality of valves (110, 220), and said distribution system comprising a first distribution wheel keyed onto said drive shaft, a second distribution wheel which is integral with said driving disc (11, 11B), and a flexible drive element which connects said distribution wheels (51, 52) so that the rotation of said drive shaft is transferred to said driving disc (11, 11B), wherein said engine (1, 1B) comprises a sleeve body which is integral in rotation with said driving disc (11, 11B), wherein said driving disc is placed at a first end of said sleeve body, which comprises a flange portion defined at a second end, opposite to said first end, said second distribution wheel being connected to said flange portion of said sleeve body. - a driving disc (11, 11B) mounted idle on one of said camshafts (10, 20) which controls said one of said plurality of valves, said driving disc (11, 11B) rotating about the rotation axis (101, 102) of said one of said camshafts; - at least one driven disc (12, 12B) which is integral with said one of said camshafts (10, 20); - drive elements for transmitting the motion between said driving disc (11, 11B) and said driven disc (12, 12B), wherein said discs (11-12, 11B-12B) and said drive elements are configured so as to cause a relative rotation of said driven disc (12, 12B) with respect to said driving disc (11, 11B) when the rotation speed of said discs (11-12, 11B-12B) exceeds a predetermined threshold, - a distribution system which mechanically connects said drive shaft with said driving disc (11, 11B) so as to cause the rotation thereof;