A friend brought what appear to be 78-79 BS-38 carbs over to try them on my XS as a test. He had spent some time cleaning them and adjusting the floats etc. The bike fired up pretty quick with a couple of backfires when going from high idle to half throttle. I went to adjust the air mix screw and found that it was covered with a white plastic cap. there was a slot that you could adjust perhaps a half a turn. I have never seen these caps before on 78-79 BS-38's . Have you ????
tim
tim