The little brass pipe on the bottom of the carburetor is the bowl overflow tube. Its supposed to have a tube attached to it to drain fuel underneath the bike if the carburetor bowl gets over filled. The two black plastic fittings on either side if the carb are the carburetor vents. Make sure they are not plugged. The round knob (about 1cm in diameter) on the choke side of the carb is the idle speed adjuster screw, also known as the slide stop. The small brass screw near the intake opening of the carb is the idle air mixture screw. There is another small nipple on the other side of the carb near the outlet side, and that is where the oil injection line is supposed to go.
Here is a link to the factory manual for your trike:
http://www.oscarmayer.net/atc/manuals/
It should give you a good starting point for the idle speed screw, and the idle air mixture screw.