Black sooty plugs = too much fuel or poor ignition. Check your fuel pressure regulator. Pull the vacuum hose off its nipple on the regulator and see if fuel comes out. If so, the regulator is knackered and you need a new one. If not, hook a Mityvac to the vacuum fitting on the regulator and see if it holds vacuum, like 24" or so. If it doesn't hold vacuum, bad regulator. If it holds vacuum, hook up a fuel pressure test gauge set and test the fuel pressure at the fuel rail and see how it compares to the manual spec value.

You do need to put the correct plugs back in it.