RMH black goop exhaust

Greetings. I'm still in the process of adding more mass to my bench but my rocket seems to be working perfectly. The only concern I have is in my first inspection I noticed a black goop accumulating at the clean out at the vertical exhaust. I also put my hand in the final exhaust cap above my roof and there was some black soot accumulating. What could this be? Signs of a serious incomplete burn? The exhaust comes out warm but not hot. The exhaust is also dripping hard. Thanks!

Another pic showing goop

Any kind of goop accumulating in the exhaust of an RMH is bad news. Obviously you have some kind of problem with incomplete combustion, but we need many more details of your system to begin diagnosing the cause. What is the character of the burn in your feed? Pictures and full description with dimensions please...

To me, that looks a lot like fly ash mixed with condensed moisture!

What is fly ash? Also realized I forgot to add the spacer like I planned! Gap was 5 inches. Put spacer in to make it 2.5 inches. Just put it together and works even better! Yay! Dimensions otherwise are 8 inch system 7x7 burn chamber 7.5 highx7. 35 feet of pipe after accounting for 3 elbows. I guess I'll let this run some more and see if I get buildup.

Fly ash is the superfine ash left over after a burn. When your mass is finished and completely dry your dripping will stop (mine took almost 6 weeks) Now that you have reduced your top gap , you will see higher temps & that in turn will help your mass dry out. Your chimney top temps should be in the 130 F. range when you get everything warm and dry. Remember that the vertical rise after the mass does not count against your 50' limit on an 8" system.

This sounds a lot like my experience last winter. In my case the exhaust exits the building horizontally and immediately Ts to a chimney. Due to a slight miscalculation the exhaust tilts downward for the last 10 or so feet of the horizontal part, and I got a large stalagmite of ice under the T. The system didn't "rocket" as much as I thought it should and often needed some help from a fan. But this year (so far at least) since I added more mass around the burn and feed area as well as using dryer wood, it has been acting much better. I am hopeful that much of the moisture I saw last year will be gone as more of the mass has dried and I'll be putting less into it in the form of green wood.