I just completed a tbi to carb swap in my 89 iroc z. my engine is just a stock 1988 "truck" Tbi 350. the motor has not been touched as far as longblock only has 70,000 miles and got it for a steal. i bought a edelbrock intake and had a edelbrock 750 performer i know its a little big but i plan on getting it tuned correctly by a friend. im using a accel hei distributor also. ive got it running i still need to set the timing gotta get a light tommarow but i thinks its really close as it idles great and doesnt miss or sputter revving it but when i let off the throttle it backfires threw the exhuast only not the carb at all. i dont see any vaccum leaks. any ideas on what might be causing this? sorry for such a long post

 

Most likely exhaust leaks near the heads and/or exhaust manifolds (collectors).

Your TBI used DFCO, so fuel was cut off during hard DECEL.