sounds like the carb might have water in the bowl from the vent lines getting under , and the packing in the muffler might still be wet. did you have water in oil? did you check .
The screw is keeping gas in why don't it keep water out?Bottom of carb, not the main gas valve. Turn main gas valve back on. Take off seat, remove right body panel. Look at carb, right in front of primer there is a regular screw. Open that screw, gas will drain from the hose you see at bottom of carb, right under the screw. This is the hose you will have to plug next time you go in.