Hi, I have a 1993 NSR 250SE MC21 with dogfight expansion chambers, derestricted, drilled out airbox. The bike has been starting and running fine since I bought it last October. But recently it bogs and stalls when I give it gas. Once underway with momentum, it runs fine...but it will stall when I try to get going, not accepting throttle. It feels like a cold bike that needs choke - - in fact, if I choke it about halfway, it runs better. Which leads me to believe it is too lean. But, how does a bike get leaner? Are there adjustments to make? Something may have vibrated out of adjustment?

Any tips much appreciated.
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1993 NSR250R SE MC21
2010 KTM 990 SMT
2004 KTM 300EXC
1996 Honda XR250