You could have some trash/dirt in your fuel tank causing the float/neddlevalve to stick , it only takes a tiny grain of sand or dirt in the fuel to cause them to stick ; what i would suggest is that you install a clear inline fuel filter between your petcock/on/off valve and carb then you may need to remove the carb and clean it once more !

And by all means do cut the fuel petcock valve to the off position when you shut down the engine , this is covered in the owners manual on all motorcycles and quads that have a fuel shutoff valve ; it's standard proceedjure on all types of internal combustion engines equipted with a fuel shutoff valve !.....

 

Sounds like you need a new needle and seat, After you get the float and needle out of the way look closely at the seat, it should be round where the needle sits, with wear they will almost look square on the inside.
As for starting, make sure that when you pull the choke lever, the choke plunger actually moves off its seat, take it apart and make sure it moves. After it is all back together and ready for a cold start, full choke and no throttle. If you open the throttle the preasures inside the carb even out and the choke does not work at all. I have even seen cases where to fast of a idle would make a engine hard to start when cold.

 

the inside of the seat wears and causes the needle to not seat properly. look inside the seat, where the needle moves up/down, if you see any sign of vertical groves, that's you culprit.