The carb bowl over flows when the float valve is not working / seating correctly (this shuts off the fuel supply from the tank).
This has been covered in the past, so worth doing a search. Here is one thread to get you started
http://www.nsr250.net/forums/viewtopic.php?t=7324&highlight=float
_________________
Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30