Then I hate to say it but it sounds like you have some type of tank issue going on and this is not engine/pipe related. In the engine runs fine, tunes fine and has no problems up until the point that you shut it off and if you let it sit a while it somehow floods then it's nit the engine doing it but something else as mentioned. The carburetor only sucks fuel when the engine is turning over which if it makes sence is pulling air over the spray bar which in essence creates a vacume pulling fuel out of the tank. If the engine is nit turning over there is no airflow over the spraybar hence no vacuum. So...to menit has to bs residual pressure in the tank forcing or actualy pushing fuel into the engine causing it to flood and flood so excessively that you hear fuel sloshing around in the pipe if i remember what you said correctly. So again this to me sounds like some type of tank related issue.

Regards,

Ron