An irrigation sprinkler includes a riser and a nozzle rotatably mounted at an upper end of the riser. A gear train reduction is mounted in the riser and a turbine is coupled to the gear train reduction for rotation by water entering a lower end of the riser. A gear driven coupling mechanism mounted in the riser couples the gear train reduction and the nozzle. A pressure regulator is mounted in the riser and is adjustable from a top-side of the sprinkler.