2007 yz 450 won't start help please out of ideas

my brother has an 07 yz 450 and the same thing is happenning. i has spark carb is spotless confirmed the cam timing is correct but it won't start. checked valve clearance and all five are right in the middle of the spec'd range. the bike back fires just about every kick but will not fire up. either very loud bang or a soft fluff of flames shoot out of the pipe. the bike was big bored and stroked and ran mint for 11.2 hours since i built it. but now all of a sudden this problem. has me stumped. i have a big bore yz 444 ported with hot cams and i built boh motors at the same time. can anyone help me out.

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