I had a similar problem after rebuilding a friends crf 450, was hard to start and was running really rough. I was getting quite worried and going over and checking everything again. All that was wrong was the flat cap that sits on the back of the carb slide was upside down. I'd cleaned the carbie when I had it pulled down and put it back in the way I thought it should go, not remembering which way it came out. Hope this is all that's wrong with your's. Good luck.