Jim looked at the pictures and thought on the second picture that you had some live coals extending onto the burner ring. Check to see if this is true. Live coals are not allowed on the ring at all and if they are, you should increase the air setting slightly to bring the ash ring back two inches from the edge.
He told about a trick that he learned 30+ years ago. After you have determined the amount of coal feed that you need for the house, you adjust the air to match the feed rate. The old timers would keep increasing the air until ash would start popping off the top of the fire and then cut it back to where it would settle down and leave the setting there. Then the homeowner would observe to see if the ash ring was about two inches wide around the edge like it should be. The proper ash ring was considered more important than the black spot.
If you add too much air, you will create more ash and the ash will start to resemble sawdust and your fire will be a little too low in the pot. You want to keep the ash ring about two inches wide and the live coals off the ring. The retired gov't lab man said that perfect air will result in an ash ring that will have some unburned specks of coal in it, or just under 100% burn.
All is well, I increased the air and now the fire looks the same except the ash ring is wider, no hot coals are on the lip of the pot. There are some black pieces in the ashes but no real difference from before. Actually except for the thickness of the ash ring everything else is pretty much the same, I didn't notice any more or less heat or coal consumption.
I am new to burning coal. From looking at these pictures it looks like my fire is "close" to this, but I don't ever notice the "dark spot". It looks like I have a nice fire, with a ring of ash about 2 to 3 inches around it (no hot coals in the ash ring). I've check the auger feed pipe and it's not warm at all. Is there anything to worry about not seeing the dark spot?
I want to verify tonight, BUT...I think the coal feed is set at 4 teeth and the air is 4 1/2 or 5. I believe my low limit is set at 140 and high 160( maybe 180). The high temp that will not allow the time to start is at 205. I have not had any outfires.