Your condition is also news to me. BTW, there is absolutely no reason to be running a Brison engine rich for break in. Tune it to provde the best performance from the first run. It will take care of it's own break in if done that way.

In the meantime, look for leaks in the fuel supply plumbing and for loose screws an retainers on the carb. At the moment that's all that comes to mind. I don't believe that pressure equalizaition at the diaphram is the issue here, but an easy check would be to simply silicone an open ended cover over the diaphram hole that has the open end pointing to the rear of the plane and the closed end to the front. Do not block the hole, though.