Yeah, pretty sure it's the secondary air shutter. Centered on the back of the stove almost at the bottom, about five inches long, one tall?Is it possible the “little air inlet door” is the secondary air shutter? This will not move with the primary air control. It’s run by a probe in the refractory box. I’m going with poor fuel or a clogged cap or a flue that needs cleaning. I would inspect the flue pipe at the 90 and also inspect your cap.