Agreed, if your filters are clean you still have air in the system. The black smoke doesn't necessarily mean it is too rich, just incomplete combustion. A tiny bubble can cause a problem, a lot of injector pumps aren't suction pumps only pressure pumps.
If you can get help and have an air compressor wrap a rag around an air nozzle and use it to pressurize the fuel tank, start bleeding at connections. It only takes a few pounds of pressure to push the fuel so don't try to make a perfect seal, let the air leak out as long as you get fuel flow.