Tom,
I've owned several (none currently) but just finished building one for a buddy of mine. He decided it was time to replace his 90 model (with 89 front end) that he had bought from me years ago. I stumbled across a 97 model that had been ridden once in the back yard and parked. It's brand new! We got it for $2199 and he asked me to go to work on it. Even with all the jetting experimentation I'd done on them in the past, I was never completely happy with how clean they ran.
I convinced him to send the top end to Eric Gorr and do a few other things to it. The way it sits now, it has the following from intake to exhaust: Twin Air filter, Boyesen Rad Valve, EG jug/head, PC Platinum pipe (taken from 90 model), stock silencer. I got the top end back last Friday and have only had a chance to ride it a little here on some level ground, but I can already tell you it's night and day different from stock. It is extremely clean running from bottom to top with no blubbering, raspiness, surge, etc. Take a corner in 3rd, roll the throttle on and it just pulls like a train, smooth and strong. All this on 93 octane pump gas!
I grafted an OEM 93 KX250 rear fender on, only requiring slotting the holes in the fender and trimming the area by the silencer with a Dremel. He sprung for a set of Pro Tapers, Scotts damper and bark busters. We have an O-ring chain on it, radiator guards and a skid plate. I also put on an extended clutch arm (10mm) that I had from years earlier to ease the pull. We plan on putting a FWW on it and we have some more cosmetic goodies for it but won't have time to put them on before this weekends maiden voyage.
EVERYTHING has been Loctited! I run different jetting than most everyone else, as I use the older CR jets (R1368N or R1370N needles), but it's really important to lean out the bottom. Even with the best jetting on a stock cylinder, I never got one as clean and smooth as this one is now.
We're in SC, tight woods but some wider trails and anywhere from 0-500 feet. The KX5's have been anvils. I can't wait to do some earth moving on this one!
Enjoy it!