21 might be too much for a stock CDI, whereas the Dyna's stronger spark can handle the bigger gap.
I'd say you're rich, and not enough timing. Of course the dome configuration is a huge variable also. What is the squish? Didn't see what you said compression was, but of course that could affect what you're plug is telling you as well.
In the original post on this thread, I came to the conclusion that it was way rich, and holds heat in the pipes, while cooling the cylinders. This is even more dramatic with the T5s. I've since learned to tune my OOF bikes after a run by feeling of the pipes, transfers, then reading the plugs to get the jetting spot on. Cool pipes/hot transfers = lean most of the time.....very HOT pipes, cool transfers = rich. I also put that one back on gas with stock carbs. It never seemed right with the 34 pjs on alky (zero port work and T5s.) I also ran/run 20:1, not 24 like I mentioned above. I mis-posted that ratio.