Yet another vibrating engine

Paul --

You do not indicate which airframe you are using.  For your background information, here is how the carbs are set-up on the RV-12 (912ULS) -- see Page 12-7:

https://www.vansaircraft.com/service-information-and-revisions/mm-maintenance-manual-4/

--  David

Thanks David, I'll check that out - I have a Zenith 750 STOL

If you don't have access to some vacuum gauges then you can get a rough idea of the carb synchronisation by measuring the distance from the throttle arm cable connection point and the bracket that holds the cable adjuster. Set the throttle in the cockpit to different positions, idle, half and full throttle and compare the measurements between the two sides. That will tell you if you have approximate synchronisation.

Based on your symptoms I suspect that you would have to be a long way out for the engine to actually quit. I would be thinking more along the lines of a blockage in the main jet, the idle jet or the very small drillings in the main venturi just downstream of the butterfly valve.

My money are the carbs are way out of sync. It could easily be the carb sync and there is no substitute for doing this procedure and should be done before any flight. Do not let a mechanic say he can sync or set up the carbs by ear or listening. If you ever just want to change the idle then gauges need to be used. 

Since it is new you may have had a piece of debris that got into a carb. Since you looked at the floats you should have noticed if the carb bowls were clean. It is possible something got into the jets, but not real likely. Just a minor likely. :) 

p.s.

I would recommend gauges over the electronic sync devices. You can diagnose and see things with gauges you can't on the electronic device.

“Oh, it definitely sounds like” ...an air leak somewhere in the induction system. But, if all pneumatic connections are secure and leak free, then step back, do the mechanical synch, then a pneumatic synch, per the Line Maintenance Manual.