My guess would be your front cylinder going...
Ive had it in the rain on my hire bike (damned thing aint got a fenda extenda) Bike idles.. just about.. at 600 rpm.. stalls if you rev to quickly... the give away was I could put my hand on the front downpipe without my hand burning
thing was just wet, and cold.
Tried strarting the thing next day... didnt dry out.. so off came the radiator, out comes the spark plug cap, and behold.. the thing was full of water.. WD40.. and a few rags in there to get the water out.. started first time after that