Agree with 650Stew, a carburetor cleaning is probably in order.
Yet, without the opportunity to examine the bike, confident diagnosis of the problems you mention is impossible.
The bike has sufficient compression to start and run; given the owner's telling the truth, chances are the problems are minor.
Even so, you'll need competent technical help to diagnose and to fix whatever needs corrective maintenance. If you can't avail yourself of this technical assistance, at least at first, the purchase might be questionable.
Again, the problems sound fuel/mixture related; draining the tank and replacing the fuel with fresh gasoline, a dose of carb cleaner, or a carb disassembly and cleaning may be all that's required; or--you may need a new carburetor diaphragm, or further carb work.
All-in-all, KLR250's remain rather bullet-proof, in my experience.
Hey, I just thought of something that MAY work!
DRAIN YOUR CARB FLOAT BOWL. Shut off the tank petcock, then there's a little screw on the side you can loosen, spilling the contents out the dump port (collect in a can or bottle, if you don't want to spill it on the floor or ground). Then, tighten the screw, and open the petcock, turning the fuel on to re-filll the float bowl.
HOW MIGHT THIS WORK? Moisture can enter the fuel system, even by just daily humidity and temperature changes. Since water is heavier than gasoline, it settles at the bottom of the float bowl. The MAIN JET is the lowest in the carb, and does not open 'til the throttle is opened, and the slide and needle rise, uncovering this lowest jet. If water exists in the bottom of the float bowl, water will then enter the engine through the carburetor venturi, and you'll have an incombustible mixture, causing stalling.
This condition is possible, and the engine operates o.k. at idle, because--the PILOT JET is highest in the carburetor; it may "skim off" gasoline from a layer above the water in the bottom of the float bowl.
So, bad or contaminated fuel may prevent the engine from operating at speeds above idle; you may fix this by draining your carb float bowl, replacing with fresh gasoline.
Last edited by LoneRider; 01-01-2012 at 02:55 AM.