Drums run 50 deg hotter in the center than the outside edge your cooking at 300 on the thermo that is 350 in the middle sugar will start to burn at 320 add the prolonged cooking time and there ya go. If your fire wasn't right you would not get to 300 so it aint an air flow issue. I use a Low sugar rub and no diffuser in My UDS and I cook at 300 all the time. this is a pic of a Butt that is almost ready to come off the pit.
