I just had to post what took place last week with my CRF250r. I rebuilt the carb last week and noticed the last owner of the bike had changed out the pilot jet from a 40 to a 42. A 40 is what the manual calls for at sea level. I'm at 4700 feet so I figured I should at minimum take it back to a 40 due to my elevation. Well, I put the carb back on and could not start the bike for nothin. Thinking I had done something wrong in the rebuild, I ended up requesting the help of the local shop here in town. Long story short-- they ended up putting a 48 pilot jet in and it runs great. Go figure. Just wanted to share so riders understand that you can't always go by the manual. No two bikes are the same.