Backfire

I have a 94 carb model of a 900 monster, and was wondering if anybody has the same problem as me... evertime it rains or gets damp (I only ride in dry) I start my bike after a few days it backfires on me, does anybody have a solution or has experience this them selves, I do not have a garage as I live in flats. There is of course no fuel tap for me to drain petrol of engine so I'm puzzled maybe it's something I'll have to live with. Any ideas??