[nkaminar]

What carburetor are you running? Here is what I would do if it was my car. I would go back to the stock spark plugs. Then I would try a known good carburetor from a friend, or if funds are not a problem, buy one. The low mileage and the black plugs are an indication that the carburetor is running rich or that the plugs are too cold. If the borrowed carburetor solves the problem then I would look at the float level in the original or perhaps a leaky gasket or other reason the carburetor is running rich.

The only other thing I can think of is that the ignition timing is off. Check the point gap and then check the timing. You don't have to change the timing to check it. If it is correct then don't mess with it. Otherwise, try a new condensor. If the condensor is marginal then it can cause missing.