Correct, there is a thumb screw with a slot for a screwdriver on the left side of the carb assembly of the left carb. Turning clockwise will increase the idle speed of that particular carb and increase overall RPM at idle. HOWEVER, you must then use a LONG screwdriver through the hole in the frame to adjust the right carb to synch them together. If they are not synched properly, you will notice that the RPMs will go up when you twist the throttle, but then stay there and come back down very slowly, if at all.
Check out page 20 of the Honda Service Manual or pages 48-49 of the CB360 Owner's Manual.