Size of Flames in a Anthracite Coal Stoker

Hello all,
I am new to this forum and to coal burning. I have found this forum to be a treasure trove of information. I have a Alaska Channing Mark 3 stoker direct vent. Usually the flames inside are around 3 inches tall, with the center of the flame a few inches taller than that. One night I noticed that the flames were much higher going right up into the stack. Is this due to too much draft? I have a barometric damper (type RC) installed and it seems to be working correctly. It is usually open about an inch more if the wind is blowing. The weight is set around 6. Any help is appreciated. Thanks.