What kind of maintenance have you done to the bike in those 10k's?
Here's the service schedule for our bikes.
http://faq.ninja250.org/wiki/Service_Schedule
Obviously yours is well above the 18,000 mile cap they have on there but that should give you a basic idea of things that need done and when they should be taken care of.
As for your problem specifically, I'm far from an expert but all I can tell you is that shimming the needles worked for me, so it may be something you want to start looking into, at least until someone smarter than I gets in here to help you out
That, and a good carb cleaning won't hurt. I imagine there's quite a bit of build up in there after 34.5k miles.