as compressed air through fluid connection 23 coacting with orifice 22 is a plenum 27 which is circumferentially bored in the side walls of body 10 forming an enlargement of the bore at a location in communication with fluid connection 23 from fig2 it will be noted that incoming air can rush into plenum chamber 27 and immediately pass toward venturi 14 through orifice 22 a second cylindrical flange 21a is formed on the yarn guiding element 12 a distance d upstream from the first flange 21 within the bore 9 of the jet body downstream of the gas inlet 23 the second flange 21a has a diameter that is less than the diameter of the first flange and preferably extends about one half the distance beyond the yarn guiding element 12 as does the first flange venturi 14 is free to move axially within the body 10 and a seal is formed between the venturi and body by o ring seal 24 seated in an annulus 25 in the body the venturi 14 is press fitted into collar 16 and collar 16 is free to move within the recess 26 at the outlet end of the jet body a circumferential groove 28 is formed in collar 16 a rod 30 extends through body 10 and engages groove 28 the rod is rotatable in both the body and the groove a handle 32 is attached to the end of the rod so that the rod may be easily rotated the rod is not completely circular but has a cam surface 31 intermediate its ends which is coincident with the groove 28 first.