A rotating stream sprinkler including a rotor plate supported on one end of a shaft for rotation, in an operative mode, relative to the shaft; a nozzle located along the shaft upstream of the rotor plate; the rotor plate formed with a chamber and one end of the shaft has a stator fixed thereto within the chamber, the fluid chamber at partially filled with a viscous fluid; and wherein the chamber is at least partially closed at an upper end thereof by a rotor cap plate; and further wherein an underside of the rotor cap plate is provided with a first plurality of teeth and an upper surface of the stator is provided with a second plurality of mating teeth adapted to engage the first plurality of teeth to enable rotation of the rotor plate with the shaft in the adjustment mode. A flow rate adjustment mechanism includes a throttle member threadably mounted on the shaft for movement relative to the shaft, toward or away from an annular seat having a discontinuous edge such that the flow rate cannot be shut off by having the throttle member engage the seat.