I would suggest closing the secondary air completely once the coal fire is established.
Coal likes all the air to come up through the coal bed from underneath. Any air above the fire is good only to help burn the initial gasses and fumes from a fresh load of coal. So once the coal is burning and you can see the blue-white flames above the coal close the secondary air vent.
I'd try closing the under-fire air some too. Unless you need a lot of heat, I'd try burning it with less air, and see if you get enough heat.
Do you have a thermometer on the chimney pipe or on the stove body?? These help a lot to determine the best burn rates. If you burn too fast, a lot of your heat goes up the chimney, and a slightly slower burn rate will give almost the same stove temp with a lot lower chimney temp.
Do you have a barometric damper on your chimney? or a hand damper? How tall is the chimney, and what is it constructed of? Masonry with a clay liner, or a Stainless Steel chimney?
Hope this helps.