Mine is a relatively new to me 99 YZ400F with a keihen carb, everything was great during the 1st several trail rides. My last ride was at +\- 6000 feet and she had trouble idling and staying running. After reading many carb post on this site, I decided to check my jetting. At the same time I replaced all the carb tubing, and purchased a new easily adjustable fuel screw. I have 170, 65 and 45 jetting, the main is a JD, not sure about the rest, original fuel screw was a red with a knurled end which was originally set at 4.5 turns out, anything less and she would not start. (I have read some comments about a JD red fuel screw). I pulled the carb, pulled the bowl to check jets, ( had to pull the 45 jet to read size), reassembled everything with new hoses and new fuel screw, started at 2.5 turners out, and she started fine, seemed to run ok ( in my driveway). I tried adjusting the fuel screw using the instructions from this site. Upped the RPM to +\- 1500 rpm and turned the screw in waiting for rpm changes. I turned the screw in all the way with only a small rpm change just before bottom. I then turned the screw out to at least 4.5 with virtually no change. I then ran it all the way back in and then out 2 turns. I have not had an opportunity to really ride yet, but can anyone explain this? BTW, home base is +\- 600 feet.