The 912ULS has two carburetors each carburetor supplies fuel / air to only one side of the engine. If they don’t supply exactly equal mixtures then the engine will run rough. This is the reason we do carburetor balancing. I believed if the balance tube was big enough there will be shared mixture between both sides of the engine and a smoother run will be the result.
I read on a few forums where people had tried many methods to increase the size of the balance tube on the 912ULS in many different airplanes. I did mine this way so the transfer of air would be as smooth as possible without any sharp corners.
I bought 2 used immaculate intake manifolds off of EBay, of all places. The tubing I bought with the 90 degree bends and a straight section for the middle piece. I machined the manifolds for a press fit to the elbows. The welding went fine, I was expecting problems with the cast manifolds. I bent the center tube by packing it with sand to keep it from collapsing.
The original couplers were made from 1″ silicone that I had used on my coolant system, after 10 hours of use they were feeling soft so I changed to high temp silicone (red).
This mod wasn’t done to avoid balancing the carburetors, I still balance them and block the balance tube when doing it.
The result has been a noticeably smoother running engine at all RPM’s.