backfiring

i just swapped from a 2.3 to a 2.6 (i swapped engines and harnesses) the 2.6 was running fine in the truck i pulled it from and i just got everything hooked up to turn it over and its backfiring like crazy. I was thinking maybe the rotors screwed up except that it was running in the other truck. What else should i check.... just so you now i am a rookie when it comes to engines and have no clue what im doing.lol