06 kawasaki 450f no start

ok so im helping work on a 2006 KX 450F and I have never ran into the problems this bike is giving me, can kick the hell out of this bike and all it will do is every now and then it will shotgun through the exhaust.
when I started working on it the cam had spun on the intake side so replaced both cams and cam chain, bore scoped the cylinder and did a leak down test and needed a top end. put in a new piston and rings and shimmed valves to spec. double checked timing and went to start it, and shotgun through exhaust again. went back over everything and valves were still in spec and timing was perfect so did another leak down just to make sure everything was good and it held back all pressure.
pulled carb and cleaned it and inspected everything in it and put back on and once again backfires.
checked stator ohms and checked out good so re checked everything again and again everything checked out. bike has perfect spark, perfect compression, perfect fuel so went back to the theory board and ordered a CDI thinking it could be messing up the timing for some reason, installed the new cdi and same thing shotgun out the exhaust.
got pissed off at the bike so took it to the dealer to see if they could help. when they called they told me everything I already knew and everything was checking out fine but could not kick start (guess they pull started it and it ran good). they thought coil so they put one on and no luck and took it back off. only theory they gave when he picked it up was that the piston may be in backwards.
got it back and pulled the head back off and just as I thought piston was in right (dot to exhaust).

anyone have any theories I havnt tried? could the tps sensor be messing everything up?