Venture sputtering and backfires

Hello.I have a 1998 chev venture 3.4L with about 225,000km
I just changed the fly wheel on it, and put the motor back in, and hooked everything up. When I started it, it idles reasonable but when I give it gas it sputters and every once in a while it has small backfires. They arent loud more like a putt. Putt. And the engine doesnt seem to have much power when I got to take off. Everything worked fine on it before the fly wheel went, so I don't know whats wrong.I keep checking to see if I forgot to hook up something but theres nothing? Is there something I did wrong or what could it be?

Thanks in advance for your time

Trevor