An internal combustion engine (1, 1B) for a motor vehicle having a ridable seat, wherein said engine (1, 1B) comprises a drive shaft and a camshaft (10, 20) which controls a plurality of opening or relief valves (110, 220), wherein said engine (1, 1B) comprises a device for changing the timing of said valves (110, 220) with respect to said drive shaft, wherein said device comprises: - a first disc idly mounted to said camshaft so as to rotate about the same rotation axis as said camshaft, wherein said first disc comprises a first side defining first tracks, each of which extends along a first reference direction; - a second disc which is integral with said camshaft, wherein said second disc comprises second tracks facing said first side of said first disc, wherein each of said second tracks partially faces a corresponding first track of said first disc and wherein each of said second tracks extends along a second reference direction which is different from said first direction; - a plurality of drive elements for transmitting the motion between said first disc and said second disc, wherein said drive elements are interposed between said discs (11, 12) and wherein each drive element is accommodated between corresponding two of said partially facing tracks (31, 32), and wherein as the centrifugal force caused by the rotation speed of said camshaft changes, each of said drive elements moves along said corresponding partially facing tracks (31, 32) between a first reference position and a second reference position which are close to and far from the rotation axis of said camshaft, respectively; - axial preloading means which act on said first disc, thus preventing a translation of said first disc with respect to said second disc along a direction parallel to the rotation axis of said camshaft, wherein said device comprises means for retaining said drive elements, wherein said retaining means (15, 16) are operatively interposed between said first disc and said second disc and exert a force which tends to oppose the movement of said drive elements towards said second reference position, characterized in that said retaining means comprise: - a discoidal element interposed between said first disc and said second disc so as to freely rotate with respect to each of said discs (11, 12), wherein said discoidal element defines at least one opening crossed by said drive elements, wherein said at least one opening defines a plurality of guide surfaces, each of which comes into contact with a corresponding one of said drive elements during the movement between said reference positions; - elastic means interposed between one of said discs (11, 12) and said discoidal element so as to exert a force on said discoidal element which keeps each of said guide surfaces in contact with said corresponding one of said drive elements. - a first disc idly mounted to said camshaft so as to rotate about the same rotation axis as said camshaft, wherein said first disc comprises a first side defining first tracks, each of which extends along a first reference direction; - a second disc which is integral with said camshaft, wherein said second disc comprises second tracks facing said first side of said first disc, wherein each of said second tracks partially faces a corresponding first track of said first disc and wherein each of said second tracks extends along a second reference direction which is different from said first direction; - a plurality of drive elements for transmitting the motion between said first disc and said second disc, wherein said drive elements are interposed between said discs (11, 12) and wherein each drive element is accommodated between corresponding two of said partially facing tracks (31, 32), and wherein as the centrifugal force caused by the rotation speed of said camshaft changes, each of said drive elements moves along said corresponding partially facing tracks (31, 32) between a first reference position and a second reference position which are close to and far from the rotation axis of said camshaft, respectively; - axial preloading means which act on said first disc, thus preventing a translation of said first disc with respect to said second disc along a direction parallel to the rotation axis of said camshaft, - a discoidal element interposed between said first disc and said second disc so as to freely rotate with respect to each of said discs (11, 12), wherein said discoidal element defines at least one opening crossed by said drive elements, wherein said at least one opening defines a plurality of guide surfaces, each of which comes into contact with a corresponding one of said drive elements during the movement between said reference positions; - elastic means interposed between one of said discs (11, 12) and said discoidal element so as to exert a force on said discoidal element which keeps each of said guide surfaces in contact with said corresponding one of said drive elements.