Is it timing, carburetor or something else?

[strummin67]

I have a '69 C10 292 4speed. Bone stock. It's never really run great to begin with but I have made some improvements over the years.

Anyway, lately it's been having power issues. It has no power until you get to 1500rpms. It literally chugs along, blowing out black smoke until you get past that point. I've played with the timing, swapped out three carburetors, including another known good one yesterday. So far nothing has changed.

I have not pulled the valve cover or the head simply because I'd like to think that the problem is external first. What should I try next?