I got a YFZ 450 and hit the dunes last weekend. It ran great, but now when I start it up it back fires when you rev it up and let it idle back down and smokes blue. I have a FMF pipe, K&N, and have a fuel pump cover. It is also Jetted at 145 and on clip 3.

Any suggestions on what my problem is ?

Good for you getting a fast quad. Whenever you rev up an engine you are creating power. Where does this power go if you are not moving?????? Do not rev up and release, rev and hold and slowly release, much better for the engine. The stock YFZ has a 155 main jet, you downsized? Made the hole smaller. Put in a 165. But your clip is low also to compensate. By running to lean you may have burnt a valve. Try the main first and see. If you have the full exhaust and or the power lid too you will have to go even bigger jets.