Contrary to popular belief spark plugs do get OLD, and I mean that even if they aren't being used. Black indicates a rich condition but you can't say it's running rich until you take it for a long drive with the new plugs in it, then see what they look like. Black THEN means yes....too rich. Gray or beige is good, white is lean. I think your main problem is a lack of use. Drive the thing more and it'll run like a top!
To me...you shouldn't have messed with the carb or the timing to figure this out. Those are set and don't change from the last time it ran well. Now you may have more problems from doing so.
