Part throttle bog then backfire?

I looked up a previous thread and saw your combo, then i looked up the specs on the new carb.......The jetting, power valve and pump squirter look pretty good to me. I agree that the accelerator pump adjustment could be some of the problem, but i'm wondering if it's not a float adjustment issue. Slow secondary opening, while you may need to play with that at some point, should not cause the issue that you describe. Make sure the pump arm is adjusted and then i'd check the float adj.