Interesting comment on oil control rings fitted incorrectly, however I would go back and consider that the fault definitely isn't any of these before a strip down.
1) carbs running rich or imballanced on 1&2 it seems too much of a coincidence to me that 1&2 fowl and not 3&4 and they are on one carb.
2) spark plug heat range, change to a hotter plug and use NGK plugs, (sorry my preference I find they are the best in my experience, neither Champion or Bosch can match them). A colder plug can easily fowl up and cause running problems. (This is an easy one to check, just get some hotter plugs of the same type, I can't recall the plug code numbers so can't post recommendation ). I have a tuned cross flow ford with twin dellortos on my Westfield, on track days I can run colder plugs as thrash it around, but around town it will play up with erratic tickover and missfire and more difficult to start as the plugs start to fowl.
3) did you change plug leads and dizzy cover?
All 3 of the above will compound each other
