coal not burning completely in Van Wert furnace

I have a Van Wert Model 600 coal furnace. I am burning hard coal. The air adjustment is open all the way but I am getting unburnt coal in the ashes. Anyone out there have any ideas for a solution? Thanks

Did it just start doing that or did it start when you got the new load of coal? For starters I would let the fire burn out and then vacumn out the grates and take a mirror and look to see if the grates are cracked or pluged. Van Werts burn buckwheat. Burning rice clogs up the grates. You can also look in thru the ash door and on the bottom of the pot there is a small cleanout door. Maybe she's full of fines and she can't breath

I guess it depends on how much unburnt, most stoves will have a certain amount of unburnt coal. usually a few small pieces.
But, yes, you may need to clean out the under the grates, holes, etc... good.