It appears that problem is with carb cables or throttle system on a handle bar.Cleaning whole throttle system may help with a new cables installed. After carbs are reassembled bench sinhronization is in order before installing carbs beck on the bike.Then proper adjustments can be done.
Hope...
You may have several problems with that carb.
- draining screw tip is damaged,
-draining screw gasket blocks screw (to tick or damaged)
-bowl seat mating surface with screw tip is damaged,
-over flow bowl tube(brass tube)is cracked or loose in bowl.
I hope this helps.
Regards Zoran