That's it. That is a knob for altitude compensation. If you ride over 5000' (as I recall) you were instructed to pull that knob out. It made the mixture slightly leaner. Later bikes like my '78 CT-90 had a different carb and didn't have that knob.



A great source of parts for these bikes is here:

http://www.dratv.com

I get many parts from them. You can probably get that part from them, the above diagram is from their website.