An image forming apparatus includes a printhead, a carriage, one or more guide members, and a positioning mechanism. The printhead is mounted on the carriage. The one or more guide members support the carriage. The positioning mechanism includes an eccentric bearing and a rotation restrictor. The eccentric bearing is connected to each guide member and rotatable around a rotational axis offset from a guide member longitudinal axis to move the guide member relative to a substantially horizontal plane. The rotation restrictor defines a first point of contact with the eccentric bearing when the carriage is in a first operational position, and a second point of contact with the eccentric bearing when the carriage is in a second operational position. At least one of the first and second points of contact is displaceable with respect to the bearing rotational axis to modify the corresponding operational position of the carriage.