Bike doesn't want to start after (or when) it rains

[UNCLE WILLIE]

agree with wolf on this ... current to run the ignition circuit has to go through the ign. switch and after years n miles the contacts burn/corrode and the resistance reduces voltage to coils-wires-plugs and when u add fog/rain the problem gets worse
however its good to check for shorted wires etc first because its easy to do . don't cost anything . and potential problem due to age
not good to let bike set outdoors even if covered up .. like a car with windows down all year