Car stalls often when I step on gas

Well I change fuel pump and went under intake manifold put in new plenum gasket to see it that would make different. It took me 1 day and have. Due to bad whether and first time doing. I notice that there where 2 bolts under intake manifold that would not get tight and seem striped. It still does same thing at start up. It spits and sputters than it take off smoothly. This happens most when I start up it up. It does start up now with out so much cranking. Maybe fuel pump needed changing too screen was pitch black and appeared to be clogged. I did not clean out tank even though it had some dark in it.

Correction, actually when I step on gas after start up it spits and sputters/backfires from engine area. With a few soft revs I then take off smoothlty.