Hello Allan

If you do not have the idle air set the exact same on each carburator it suggests some problem within the engine itself.  The factory 1.5 turns is really the best even if it appears a bit on the rich side.  Consider the fact that a CV carb has no accelerator pump, it depends on some richness to pick up the throttle from idle into mid range.  

As for using ultrasonic cleaning, I am not a fan.  There are many passengers in the Bing that rely on sealed drill passages from the manufacturing.  They are plugged with pressed in balls and clearly can be seen from the outside.  To ensure there is no leakage they additionally place a drop of epoxy sealant to the ball.  Ultrasonic cleaning can cause these sealants to come off resulting in strange issues with the carb bodies that in some cases can only be fixed by complete replacement.  Just my view.

Cheers

I'm not a fan of ultrasonic cleaning either. It has caused problems for owners over the years. If your idle air mixture screws on the carbs are both at 1.5 turns out then that isn't the problem.

Have you found out,

I have the same problem in my ul engine.

I am wondering if it could be dog gear wear, not open the gear yet. 

Hi Allan

Í have the same problem in my 912ul 

My next step is to check the gear.

https://youtu.be/iJJ1nX74SYI?si=YgIdmU8diwEakAan

Try having your propeller dynamically balanced😈