I had a similar problem with my 92. Turned out that when I rebuilt it, the ring didn't seat, so compression was a little low (150psi @ 3200 feet). This caused it to run pig rich, and I was fighting jetting issues big time. Because it was so rich, the power valve began to get sticky in just a few hours.
Slapped in a new ring (now 185psi), cleaned the power valve, and wala- much, much better. The mid's still a tad rich, but it's in the ballpark. New needle should make it perfect.
Your friend's 93 has a Keihin 38PJ, much better than the Mikuni that my 92 has. Check compression, then pull the little preload cover off the powervalve to make sure it responds right away when snapping the throttle.
Stock jets on his bike should be:
Main-180
Needle- R1471NS
Slow jet- 45
Air screw- 1 1/2 out
Float height- 16.0 +/- 1mm.