912ULS Won't Idle Without Choke

Make sure the mixture screw is at 1.5 turns out. This sounds like your idle jet(s) are clogged. Easy to remove and clean. Call me if you need help. Roger @ 520-574-1080 in Tucson, AZ 

Thank you, Roger.  I also removed the idle mixture needle valve.  It appeared to be clear (I could shine a light through).  When I put the needle valve back in, I went in all the way and then backed it out 1 and 1/2 turns.  Is there another jet that I need to inspect?  I will call you later today to discuss.  Thanks again!

Have a look at this article: 

https://electricmotorglider.com/2017/04/11/bing-64-cv-carburetor-idle-circuit-part-3/

"If the engine runs with the choke partially on, but dies as the choke is placed in the off position, it is an indication that the idle circuit is the culprit. It is absolutely essential that the idle circuit be set up and functioning properly. We use the idle circuit on every flight, and it is a surprisingly important system within the carburetor.  Aside from the practical aspects of having a properly operating idle circuit, there are many correlations with the idle circuit malfunctioning and other engine problems, ranging from increased maintenance to engine stoppage and even engine failure."

Seems to describe your situation exactly.  Did you have a look at the two bypass holes?

Thank you again Roger and Todd.  An update:

The right idle jet was clear.  I ran it again after reassembly, same problem.  So now, obviously, I'm suspecting the left carb as the culprit.  But why the disparity in EGT?

So I first troubleshoot the EGT and find they are indeed swapped - at the back of the Dynon unit.  Near term, I swapped spade connectors at the engine so that I have correct indications (this conflicts with wire markings on the engine harness, but now I'm sure the left probe is indicating Left, and the right probe Right on the D180).

Then I removed the left idle jet and found it clogged - with what, I don't know. The bowl was clean, no debris in the bottom (thankfully).  I carefully tried to unplug the idle jet with a nice, soft strand of copper wire (very thin) onto a white piece of paper so that I could tell what the debris was. No luck.  I carefully blew it out but still was unable to capture the debris.  When I reassembled everything, it now idles much better, almost correct (1700 rpm when warm) - although a little rough (see below) The carbs do still need to be balanced. And both sets of floats weighed 6.4 grams, well under the 7 gram limit for float sinking concerns.

Now I have a significantly higher drop on B ignition, 250 rpm and it is a little rough compared to 150 rpm dorp on A, relatively smooth.  I'm sure my spark plug gap (new plugs at annual condition inspection) is set at .025".  I was meticulous about this. All the plug wires are seated good.  Next step will be to pull, clean and test each spark plug.  That has to wait for another day.

Jerry,

you will find the time to sync the carbs well invested. Since you found the cause and fixed it syncronising the carbs will most likely eliminate the remaining roughness. It does make a difference.