Brett wrote:
Guys I am thinking now that it is running lean in the low rev / idle circuts when it heats up due to an air leak (hopefully not a crankcase seal) that opens up with heat and leans it out- explains why throttle is slow coming down from rev at idle and why I cna't save it when it starts to die by giving it some revs

Sounds much like the symtoms I had due to a dead crank seal, revs slow to drop to tickover from around 4000 RPM, also very eager to bog and die when opening throttle sharply at tickover, much clutch slip needed to stop engine bogging when setting off.

Try Andy's described method with spraying carb cleaner, if that's inconclusive suggest you conduct a crankcase pressure test, before you think about splitting the cases.

Some info
http://www.nsr250.net/forums/viewtopic.php?t=7023&start=0
_________________
Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30