i have replaced everything on his motor but it still is back firing and cranks whn it wants to

Don't know what the answer is

Such a wide open problem and so many components replaced based off of guesswork that it's going to be quite difficult to find out what's going on. For backfiring, I would look at engine timing. I would also check for air intake and vacuum system concerns, crankcase ventilation concerns. Also check for a clear exhaust system with no restrictions. As far as "cranks when it wants to" -- no idea; would have to catch it in the act and see what is happening when it does it.