Using that rule you should have a 3.5" pipe at 18" long, my guess is that your chimney may be to long and narrow to promote the temps that your looking for.
Don't forget the gases will cool as they rise and cool exhaust will restrict the flow.
Interesting. Yes, the tank is 18" in diameter. I will keep that in mind as I continue to perfect the smoker! The exhaust is actually 25" tall but I have been using a 5" extension to make it 30". I may take off that extension for my next smoke.
Imitation burnt ends from short ribs is such a great idea, and it looks damn tasty!
Just now got around to reading the last few days of posts, and seems like a smaller fire (less fuel) was the ticket.
Starting with a half chimney of charcoal is also a good way to get up to temps prior to adding wood for a clean burn.
Smaller fire was the ticket along with letting the log catch fire for about 30 seconds before closing the door. It ran sooooo much more efficient. I used a total of 3 logs for the 7 hour cook. Before the changes, it would have been 6-7 logs of white billowing nasty smoke. It all makes sense now....but I am sure I will have more questions along the way. Pork butt next. I have to redeem myself from the one I cooked last week for a friend and his family that are in a time of need.