Black plugs, chugging (!), & black smoke suggest too much fuel...
Presumably you know what you are doing to be rebuilding this engine?! By the way if you're jump starting when the battery is flat it's very likely the fuse on the OVP relay (sigh) is blown, which causes at least hard starting and poor running.
Perhaps some more information would be useful?!
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black