You are viewing the left side of the bike. Find the choke and look down and to the right in this picture. Right at the bottom rear of the carb bowl you will see that black knob hanging off of the bracket that is attached to the carb bowl. It is hard to see in this picture, but you can discern it as it seems to touch a black wire/tube on top of the engine case. Right behind the chromed left cover's 'tab' I think you need to turn yours counterclockwise to drop the rpms. Not too low as you want to have adequate oil pressure at idle.
PS: When synchronizing the carbs, you will want the bike to be idling at 1500 rpms.
