I had this one posted on the Southern board but it was suggested I put it here for better results. Sorry for the repeat.
A while ago I purchased one of those small grills that are shaped like a cylinder on end, with the pans for charcoaol, one for water, a door for putting in more fuel and two meat racks. With that, some hardwood charcoal, wood chips and good instructions I got off the net I thought I would be able to do some real cooking. My problem was even though I controlled the temperature around the meat (pork ribs and beef brisket) at 250°F, after about 3 hours at this temp it became very charred on the outside and over cooked on the inside.
I watched the temperature closely, my thermometer was correct, I did not use any sugar in my marinade or the dry rub and the meat was all on the highest rack farthest away from the fire. All my recipes say the meat should cook for about 6 hours and not to let the temp around it get below 250. As it was the stuff was almost inedible, if I let it go any longer I could have used it for charcoal on my next effort. Does anyone have any ideas on what went wrong?
Check out the book "Serious Pig" by John Thorne. He talks about his trial and error success using a smoker, which sounds like the device you are using.
The only other thing I can offer, is that you want to keep the heat really, really low. You gotta cook BBQ low and slow.
The water pan should have buffered the meat from charring - unless it ran dry. Do you keep a thermometer on the rack next to the meat? Did you calibrate it first? It sounds as though your temp was above 250.
If you haven't, check out the link below - it's one of the best BBQ resources on the net.
Good luck, and keep trying - Q is as much art as science!
I'm not sure about pork, but brisket suffers significant moisture loss when the internal temperature gets much above 170F. In addition to trying lower heat, you might consider using one of those digital thermometers that you leave in the meat and program to beep at a certain temperature.