I keep my bike in the shed at night and when I start it in the morning, its fine. But if I go somewhere in the rain and try to start it about 1/2 an hour later, it becomes a pain to start again. Its my only form of transport so I have to ride it in the rain(unfortunately).Maybe because when it rains it's typically colder than usual?
well I think its just moisture getting into the bike (spark plugs, air filter etc.)
When you start it, use no choke then gradually add choke until the bike starts. Make sure you let the bike warm up to get rid of the water otherwise the bike could die on you after a short distance