My Mark III will do the same thing if I load it too full all at once and it
isn't really roaring.
I have a feeling that it is worse now that I "sealed off" my window
for secondary air supply. (ahah, maybe that's why it's there)
anyway... without that air supply coming in around the glass like
it did when I first changed the glass (and used the gasket on just the
sides as instructed), I am noticing more tendency to backfire if I'm
not careful when I load. Last nite, I was teasing it, it was loaded..
ash door open, sort of smoldering, I'd close the ash door and open
the coal door, just a little, and close it.. then small "whoosh", ignition,
but with the door closed, it'd go back out out. I did it a few times, (yea,, it was the pyro in me coming out). But, I really think that if I was to
pull that extra gasket material out from top and bottom of my window that I would see ignition quicker. I just hate the thought of how the stove
burned when it was like that last month. (very hard to regulate, and
I had a few of these last winter, and was nervous at first, but after a while I learned that I could minimize it if I spent the time. Usually, it
happens with a rush load, without the stove really going.