Camshaft question

When I put my 318 together it was sending the plugs black even though I had the mixture screws in the best spot. I had idle timing set at 12 degrees. It wasn't until I adjusted the idle timing to 20 degrees that it cleaned up the plugs and made the low end response a lot better. The camshaft is a 204/214 @ 50 thou , so not a big cam.