backfiring in the TBI

I recently baught an 87 2WD suburban 454 TBI, it ran ok but was a little sluggish so I decided to give a tune-up. New plugs wires cap, and rotor went to check the timing it was set at about 12~14 BTC, spec. is 4BTC. Since then it backfires throught the carb when cruising at around, if idling or punching it seems no problem.??? Need help I'm a previous Ford owner so please help me stay with Chevy! Please! :banghead: :banghead:

I had the same problem..took it to mechanic and it was set to 2BTC and they increased it to 4BTC...it's fine now