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Where is your exhaust in relation to the opening from the fire chamber? Maybe the heat is coming in and going straight out the other side without having time to build up heat.
And does the fire chamber have a intake damper? If not your fire will be too small to make sufficient heat.
It is hard to tell from the pics, but it does not look like you have a grate in the firebox. You need an airspace under the fire to allow it to breathe. This will also allow ash to fall out and not choke down your fire.
The air intake looks like it is at the same level as the wood. You need some air intake under the fire to get it to burn better.
Your opening looks to the cook chamber looks small too. It is hard to tell this from pics though. I would correct the other two things first to see what happens.
I made a smoker with a damper on the exhaust and I have learned that with the damper closed it actually cools down. It is all about airflow. I thought that if I shut the damper then more heat would build before exiting, but the opposite is true. Maybe you should open the exhaust damper all the way and see what happens. Next I would crack the door open to let the fire breathe more to see if your intake damper is too small. Or maybe it needs a fire grate like earlier stated. I would try these two simple fixes first.
If neither of those work I would question the rectangular pipe you have going from the fire box to the CC. It looks a little small to me..
Hopefully it is the two simple things I suggested.
I agree with both Rassimo and smokinaces, you need some sort of grate in the firebox to get your fire off the bottom. Start it up next time (after a grating mod for the firebox) with your firebox intake fully open and the cook chamber exhaust fully open, see what that does for you on temps. Should fix the problem, if not, I agree that the next problem could be the choking down affect the firebox to chamber tube could be causing.
Try this and then the exhaust damper next. Hopefully this will eliminate your heating issue. Other wise it could be something more serious.
If you put a damper on the exhaust make sure to play around with it before determining it didn't fix the problem. Fire it up with intake damper wide open and exhaust damper wide open. When you get a good blaze going, shut your exhaust damper half way and let it burn for 45 mins to an hour. See how your temperature acts. If the temp doesn't level off try opening it or closing it more to try and get results.
On my double 55 gallon barrel smoker, I can set the intake damper, firebox to CC damper, and exhaust damper just right and it will hold 200* for 12 hours easy. Plus or minus 10*.
If the fire grate doesn't help, I think the dampers will help dramatically.
Made a fire grate raised 45mm and as you said with the damper fully opened i was getting tempratures exceeding 270 degrees so thanks everyone for all your help still a lot to learn and a lot of questions to ask once again thank you all.