conventional uniform diameter conduit having a length of 950 mm and a diameter of 25 mm the chain line shows the flow rate through the secondary air supply conduit of the invention comprising a larger diameter section 4a of 650 mm in length l 1 and 25 mm in diameter and a smaller diameter section 4b of 700 mm in length l 2 and 15 mm in diameter it will be noted that in one conventional supply conduit the frequency of resonance f is increased by reducing the conduit length thereby ensuring the secondary air will flow at a higher engine speed region as shown by the broken line however the overall amount of the secondary air of this conduit is not adequate in another conventional supply conduit having a greater length the flow rate of the secondary air suddenly drops as the engine speed exceeds about 2000 rpm as shown by the solid line to the contrary as shown by the chain line the supply conduit according to the present invention does not undergo a sudden drop in the flow rate when the engine speed increases and is capable of supplying an increased amount of secondary air throughout a wider.