Hi,
I have known the crankcases to fill up with fuel that has leaked past the float valves and run into the engine.
You should always switch the fuel tap to off position when storing the bike.
The easiest way to check for fuel in crankcases is to remove the carbs and reed valve block and look inside?

The carb's may need a complete overhaul and possibly the fuel tap is leaking past in the closed position.
Regards
Ian