Sean....lots of comments on the mount from me. We however have only speculation as it requires some testing for the signature for the movement in the mounts and how well it damps the vibrations.
First, your mount is not dynafocal, it lacks any design to absorb the movements in the direction of tension and compression. The Barrey mount ideally should be used in a push-pull configuration to best absorb the frequencies. This design is not really good at working in shear. This type of Barry mount comes in different hardness, generally a soft, med, hard range to select from. This would be my first preference to work with given anything else requires redesign and that gets to be a lot of work with limited return.
I can only give you an example that we worked with many years ago. The original Kitfox model 3, where they really started to use the 4 stroke engines a lot, had a lot of issues with the vent tube ejecting fuel especially at high idle and even up to about 4000 RPM. (see the SL on vibration ranges that are the most difficult) They also had issues with spreading of the manifolds to clear their special mount that exacerbated the problem. Switching to a harder mount solved most of the problems. Eventually the solution was a complete mount redesign as seen in the model 4 and 5 and up.
When Kitfox changed from the 80 HP to the 100 HP then had problems again. In that case the quick solution was to clamp the mount harder, in effect moving the resonance absorption to another area not so sensitive. inside the Berry mount there is a sleeve, we would remove it and cut them down as I recall by ⅛ or a bit more. This got us past the vibration areas that would toss fuel, again it was a factor of trying to damp the movement in that critical area. As you have noted when the carb balance is good the problem is not as bad. This does not mean it is gone so I am aware you are doing all the normal things, try working with what you have too see if you can shift that vibration away from exciting the floats, float arm (not armature, lol). What Roger suggests is working with your needle seats to be positive they are sealing as well as they can. If you think back you recall how Rotax historically has 3 different versions of the needle itself, in that they had silver, black and not gold pins on the needles. The changes in the spring internally were part of that damping of the resonant signature that can attack the parts as you have noted. Making sure you have good sealing is important but do not chace it forever, work with your mounts to see if you can shift it.
Cheers
