www.mikuni.com/pdf/tm33.pdf this shows a tear down and parst listing of that carb, the link might not work, you might have to type it in manualy, my computer skills are preatty poor. I just typed in mikuni 33 pumper carb, and this was the first thing that came up
Just looked at that again and its a drain tube for the bowl. the bigg black one on top is the throttle/idle adjustment.