I have a GT250 also. Recently, it decided to backfire alot, die, and not restart. Someone told me to disassemble it and clean the oil lines - which may be clogged due to not ridding it durring the off season. Bought it at a garage sale, I´m thinking it has never been serviced for 30´odd years since it has low miles. What you mention about misfires in the rain sounds interesting, although it wasn´t raining when my bike acted up.