You are building up carbon on the plugs causing the engine to miss fire. It is obvious your mixture is too rich. You need to fix a carb. that will run the correct mixture. I would mess with the original model B. They are simple & easy to work on. The usual problem with them is the power valve staying on all the time. There is a lot of posts on this in this website. Do a search. It would pay you to get a cheapy spark plug sand blaster & clean those plugs up instead of buying new ones every time they get carbon on them. You need compressed air to use it.
HEI would help as it fires a dirty plug better then conventional ign. That would not solve the problem though.
George
