yz motor guys need help please!

[chopp124]

i have been running these motors since 99 with the 400. know them top to bottom. however i am stumped. i put a whole new yz450 motor in my bike with valves ,piston,crank etc. carb was fine with old motor. now the second i hit the throttle the pipe explodes out the tail pipe and the coupling and shoots flames. i have swapped out electrical and carb but no luck. tor the motor down twice and everything is in spec and timmed right. wtf. im not loosing coolant but does anybody think its possible to suck crankcase pressure into the cyl through the head gasket? also im getting a mist back through the carb when reving. was told it was normal but have never noticed it before. noticed smoke out my vent as well. i know some is suppost to come out but seemed alot. compression is good and valves dont leak. any help would be awesome

[jrspawn]

How are you checking the timing? Do you verify both flywheel shows TDC and the piston itself is TDC?

Also not sure by what you mean by "the pipe explodes out the tail pipe and the coupling and shoots flames". Is it backfiring, or did the silencer blow off?

[chopp124]

feels like its firing in the pipe and its shooting sparks out tail and out coupling in between head and tail. it happens right when i hit the gas and up to full throttle. its not just when i let off. im going to start over in am with a new head gasket and go from there.

[chopp124]

piston and flywheel is both tdc. strange thing though i put the exhaust cam to wr timming to see what it would do and it wount start.wount even pop. get absolutely nothing. would think it should start that way. im going to check the cams with the crank in the down position as well. i never new to check it that way. always just set it with the marks. i have had this bike since 99 with 4 different motors. never had a problem like this before