Just a comment I'm throwing out there.......
I have the understanding that by closing the top vent, you're creating a "log-jamb" with the smoke, which backs up inside the smoker. This smoke, not being able to exhaust itself to ambient air, begins to get stale and "burnt" since there isn't a fresh supply of new air coming in the bottom vents.
That is why my top vent is always open....right or wrong...
A man wearing a helmet defending his country should make more money than a man with a helmet defending a football.