Small Block & Big Block Chevy Specific - got fuel and spark wont start

my friend just set switched from carburator to tpi he has spark and fuel and they swear the timing is right... but im making them recheck again tonight the motor is not running for anything. It does back fire and shoot flames out the carburator how ever i believe they have the timing off... and still having the same cam for the carburator for the tpi willl that matter? Anyone and ideas?

Well 1 of 2 thing.... Cam is wiped (one of the lobes is round off) or the timing... its prolly 180 out. Just gotta play with it...

most cases when it backfires through the carb then its 180 out...