At this point I usually place a screw driver with a well insulated handle into the spark leads and watch the spark through that too - to check that the quality is not being diminished by the plug. But given that you have already changed the plugs, I figure that wont make a difference.
For what its worth, my '84 B2 need all the electrics replaced on it when I got it - the previous owner had driven this thing into the ground so I know how you feel. I had the plugs, leads, rotor arm and coil replaced. I then had the idle changed. In an effort to overcome the poor electrics, some and some point had cranked up the idle speed. This in turn caused the carb to get real clogged and dirty......
Have you check to see if it has been 'firing' through the carb at all? as well as the exhaust.
Either way, I would check the rotor arm (my memory is they can break a lot) and the coil (or whatever its called) that provides the charge for the spark etc.
Again, Im only a beginner at this stuff - so I may be talking crap. Either way, the guys here have an astounding knowledge of the details of the B2
