Quadrafire Transition fitting

I have a mid-80's Quadra Fire 4100 that is running out of air when I close the main damper. I have tuned up the stove all I can, everything that I can reach is clean and or replaced, including stovepipe/chimney/topper, brick, door gasket, under stovepipe in the roof of the stove, and wherever I can run a shop vac inside and out. It is the main damper lever that is center/front on floor level of the stove that is closing off the air flow when I pull it all the way out, both dampers operate smoothly and easily. Any ideas out there?

Seems to be a matter of simply leaving the damper open a bit when firing.
When you start a fire the damper should be all the way OPEN.{inwards}....throttling it down [pulling] as it heats up.

PaulAnton.