Abstract: A mixing of mixing machine comprises a mixer body having a mixing chamber and a material inlet through which a material to be mixed is loaded into the chamber, a fluid-operated cylinder having a piston rod on which a floating weight is mounted for movement together with the piston rod between lifted and lowered positions, a support structure for supporting the cylinder for movement between an operative, in which the floating weight is in register with the material inlet, and a stand-by position in which the cylinder with the floating weight held in the lifted position is located laterally offset with respect to the material inlet, and a drive mechanism for moving the cylinder between the operative and stand-by positions.