On every charcoal smoker I've ever used, I have at least as much exhaust as intake air and control the heat with the intake. If you're putting food on before you have a clean fire, or if the smoke is trapped because there's not enough exhaust, that could explain bitterness.
Can you post a pic of your UDS?
"In order to make delicious food, you must eat delicious food." -Jiro Ono