If I may inject a couple of thoughts here, I have to agree with Mr Chad Johnson's comment about the appearance of your carb indicating previous backfire. The black soot is indicative of backfire. Engine backfire is, in my experience anyway, most likely the result of ignition system or timing problems, so I'd suggest looking at the ignition as the primary source of the your problems. From your previous posts with your "on screen" examination of the distributor (very clever, by the way, to use your digital camera to post photos real time for the participants), it sounds like the distributor primary function is working (presence of sparking from rotor to distributor cap) and the centrifugal advance mechanism is working, or at least is not frozen. I infer this from the fact that you can rotate the rotor slightly with the engine at rest; it should rotate approximately 10 to 12 degrees (translating to 20 to 24 degrees at the crank, since the distributor is geared at ½ the speed of the crank). As for the ignition box, MSD has a checkout procedure on their website for performing rough troubleshooting on their system; perhaps that will give some clues if the problem is in the ignition box. Check them out at
http://www.msdignition.com/ and look under the Troubleshooting Guide. Also, don't hesitate to contact the folks at MSD to ask for help. I had problems with my MSD distributor some time back and they were great about fixing it.
However, the backfire will blow the power valve in the carb if you do not have the blowout ball check valve installed in the base. This will lead to the further problems which you have that are evidenced by the black sooty plugs. The new Holleys, those less than about 4 or 5 years old, all have the ball valve installed. Older ones did not but can be so modified. However, from e-mail discussions I have had with Holley tech support, presence of the check valve is not a 100% guarantee that a backfire will not still blow the power valve. If you're so inclined, Willys and Moroso manufacture power valve checkers, which when used with a small vacuum pump, will check the condition of the power valve diaphragm. Jegs carries the Moroso unit (listed in their carburetor accessories section), but it is functionally identical to the Willys tool which I have. It's about $30, and I know new power valves are only $6, but it sometimes help to be able to determine the real root cause rather than change parts and then wonder what really fixed the problem. Also, Holley sells a nice little red book which explains the operation and maintenance of the carbs; it's available at most speed shops. I sympathize with you, buddy; I've had to work through similar problems and it's a bear.