I just bought a 2004 crf250r thats bored out and stroked to 305. It has the power max kit. It also has an acrapovitch full exaust system. It is a PAIN to start. Before i go and get the valves done and and carb tuned i am asking here. It back fires but at every track every time i get behind a crf250r they back fire on deceleration.


Next up was jetting, I did end up swapping out a needle and main jet. Still nothing
Then I finally figured out it was my starting technique.
Now I generally don't have any problems starting it. 

And I swear it's also part mental, so in your mind know that it WILL start.