I had the same problem with carbon on all six plugs. Replaced the coil, plug wires, cap, rotor, points, 4 different sets of plugs and still had carbon/black smoke in five munutes or less.
I then tried using Champion J-12-Y plugs. Carbon went away and engine had more power. Problem solved.
Also, like Wrenchbender said, make sure the insulator/ base gasket has the hole and is lined up correctly.
When adjusting the mixture screw, with the engine running, try turning the adjusting screw in almost all the way, the engine should stall. If it doesn't then you have other problems.
