Why my truck do backfire hit 2.5 rpm

What is your question?

I put new timing belt on it ( still backfiring most time in past) and reduced backfire put new wires and spark plug and air filter. And i cant find anything helpful hints. I try speed up but it left like so heavily get up and go in 3 gear above. I already rbuild transmission stick shift and still no different and shop check my muffled and said it fine not problems but got be somewhere it not putting more power into it?