My best advice is to always run your intake and outtake (smoke stack) wide open. Manage your temperatures by the size of your fire. That way you'll always run a clean fire and never "over smoke" your meat.
I think people get too hung up on a fully sealed firebox. Take a look at how they run the big pits in Central Texas like Kreuz, Smitty's and others. The "firebox" is just one big ass opening where they burn the coals. Nice and clean.