To provide an assembly for preventing an unprepared movement of a fuel calibration setting needle.

This assembly adjusts the air fuel ratio of an air-fuel mixture for an engine, and has a main body and a needle valve element. The main body forms a fuel passage, a needle orifice, a retainer receiving seat and a receiving hole having an inside screw. The receiving hole communicates with the fuel passage. The needle valve element is housed in the receiving hole, and has an outside screw part, a head part and an intermediate part positioned between these parts. The outside screw part threadedly engages with the inside screw part. The needle valve element advances-retreats to the orifice, and increases-decreases the fuel passage area between a needle and the orifice. The assembly has a retainer. The retainer is concentrically arranged around the needle valve element between the intermediate part and the retainer, and is compressed between the retainer receiving seat and the intermediate part, and biases the outside screw part of the needle valve element, and keeps the needle valve element in a prescribed position by prohibiting a movement of the needle.