Nothing like a 7 year old post to save the day for me. I had a similar problem described here. Bike had been cracking and popping for some time. Cleaned the carb, discovered the pilot screw had fallen out, so put in a whole new jet kit. Once I put it back together, it ran terrible. Would not idle no matter how high I set it, still cracked and popped, difficult to even get started. I tore it back down twice and consulted my expert friends before finally typing "crf450 wont idle" into google. Low and behold this post appeared, and the light bulb went off. I went back out, pulled out the slide plate, and sure enough, it was upside down. Darn thing sure looks like its supposed to go that way too. Buttoned everything up and she fired right up, all problems gone. Thanks forum!