A tilt adjustment device for adjusting tilt of a left guide bar with respect to a right guide bar, and includes a holding seat and a cylindrical-shaped bushing disposed rotatably within the holding seat. The bushing defines a bar-retention hole to permit extension of the left guide bar theretrhough, and has a turning portion defining a first axis and an insert portion that extends eccentrically from the turning portion and that defines a second axis parallel to the first axis. Rotation of the cylindrical-shaped bushing relative to the holding seat results in a radial movement of the turning portion, which, in turn, moves the first axis toward and away from the second axis of the insert portion.