I just tried out my "new to me" brinkmann gourmet charcoal smoker and it has been going strong since 6pm (now 10pm) above 250F. It has all the standard mods, three openings on the bottom. One ball valve and two straight with caps.. Also has a grate for the coals to sit on so they don't smolder them selves after long burns.
When I first started it was holding at 318F with all the valves and ports closed, now at 10pm it sits at 271F with everything closed. What am I doing wrong?