Smoking issues

I've almost completed my first rocket mass heater. 6 inch j tube with bell bench. In general I'm really happy with it. Thing is I almost finished it, got cob around barrel and burn tunnel etc and put the slabs on for the bench. Then I got COVID and have been unable to do anything for almost three weeks now. Anyway having the rmh up and running and keeping the house at a constant heat has been great. Coming down in the morning and still having 20c is beyond my wildest dreams of how well I thought it would perform. It did smoke the first few times on start up which I was prepared for but since it dried and I've been lighting it twice a day for a couple of hours there has been no problem.
This evening however I lit it after a break of 24 hours and it smoked again.. I don't know if it will get better when I add the thermal mass ( plan is another 4inch cob on bench and slightly more around barrel) or whether I have asked too much of the system by adding a long 'flue' 'hypocaust' 'exhaust chamber' to evacuate the bell. As you'll see in the photos it runs the length of the bench.. would it be a good idea to open her up and maybe reduce that length by half before it becomes an egregious job once cob is on??
Hope it's clear enough from the photos whats going on...