That flap that opens up just above your firebox is indeed a damper. Make
sure it is fully open.

Try this the next time it smokes: Open an outside door or a window (in
the same room as the fireplace) about 1/2 way, and watch to see if the smoke
goes away. If it does (and I bet it does), then your house may be a bit
too airtight, and as another poster said, the fire has to get its air
**somewhere.**

I have also found that my fireplace, which draws well, will often smoke
when I first light it up, but after the flu has warmed well with a good fire
(perhaps 30 mins or so), the smoke will clear away. On a first fire, I
frequently have to open a glass sliding door halfway or even fully open, for
the first 30 mins or so.

BTW, always make sure you have a flu inspection at the beginning of each
season, to make sure you have no blockage and no soot buildup, either of
which could cause a fire.

Hope this helps !!

--James--