[Gaz64]

I agree with a lot of comments from others, but to me, the porcelain colouring up after 200 miles says your rich.

Don't go by porcelain colour too early, you should be looking at the outer shell and the ground strap.

The outer shell for A/F ratio, and the ground strap for timing and heat range.

Get hold of a wideband A/F ratio gauge with data logging capability, hook up the necessaries like rpm, tps, map etc.
You can do all of this even if you run carburettors.