After rebuilding the burn chamber twice, the feed tube twice, the heat riser twice with no success I finally tore into the cob covering the exhaust flu and shortened the flu by ten feet. Same results and a whole bunch of frustration. Because I couldn't think of what else to do and since I had the flu pipe all exposed I tried a burn with just the first five feet of flu pipe attached. Immediate success but since the smoke was now come out inside the shop I jury rigged a pipe to my through wall exit and immediately the smoke started coming back up through the feed tube. I had used a dryer cap to finish the end of the flu. I had removed the trap door portion and had covered the opening with 1/2 inch screen. As soon as I turn the open up so the smoke could exit up the stove took off and has burned perfectly since. Apparently the angled shed portion acted as a constriction - guess I should have seen it but I sure didn't.
I will start putting it all back together tomorrow and will test as I add back in flu pipe but should be able to get there now. Sure was great to see the fire rocketing away just like in the videos I seen of all your stoves.
Hope this helps some one to avoid what I went through - Jon
Great point. A lot of chimney caps and vent covers that are designed for fan-powered equipment can be a horrible flow restriction for a passive-draft system like a rocket mass heater.
Even the small-mesh screen on a dryer vent can be a flow restriction; we usually screen with 1/4" wire mesh to reduce the friction and allow more flow area, and sometimes we make the screened opening bigger than the chimney (by mounting the screen like a perforated cylinder or cone, instead of a circular cap).