from the Koker manual, pg 14:
IF FIRE STAYS LIT, BUT STOVE IS TOO HOT
If convection blower cycles on and off often and produces too much heat, ether the fire bed is too long or timer is running too long. If you reduce coal feed or remove timer pins, do not make radical changes. Reduce coal feed 1 or 2 turns OR remove 1 pin from timer. Then wait several hours before making any more reductions. A sudden radical change may be too much and cause fire to go out.
Once the coal feed and timer are set and fire stays lit, without convection blower running too much, it is usually not necessary to make any more changes.
How large is the hot-coal area in your furnace?
I also wanted to point out that the direct vent does not have a barometric damper. It appears to me that the instructions in their manual on page 3 are for the naturally vented Koker. To change the draft on a direct vent (from the manual)
slide adjusting rod (located in rear of venter) in or out to obtain proper draft. Secure adjustment rod with set screw.
To reduce the draft you slide the rod in (close it) a bit.
My advice here is don't mess with it.
I'm curious about your plenum. Could you post a photo?
I'm assuming you have the plate across the front top edge in place and sealed off. Without this plate in place you'd be quickly blowing all the hot air out of the furnace into the room it's sitting in.