That's pretty decent for that cam, so I'm guessing your timing.....initial anyway is "close". From your description of the black smoke, I'm guessing the carburetor "has issues".
I would try a step or two lighter springs on the metering rods. If it gets better, you're "going the right way". BUT, there may not be enough "room" with just spring changes. You may need to run bigger metering rods. May even need to re-jet it. Between springs, rods and jets, I don't think there's any doubt you can get it done.
I do think that an A/F meter will help you greatly in this instance. Worthwhile investment for sure.
