You have covered all of the basics.
Spark plugs are telling you that you have a rich condition. An engine can run pretty good even if it is running a little rich.
I am guessing that you are having more than one issue at the same time. This makes troubleshooting difficult.
My bet is that you have an ignition problem, such as dirty/pitted points and/or weak condenser. The points are a year old. Change them and the condenser. They are cheap. Make sure to burnish the new points with a credit card to clean the crap commonly found on new points these days.
Once you are done with that, it will be easier to track down the cause of the slightly rich condition.
