Not being familiar with YS engines and you didn't state which plane
you had it mounted in, I would guess that the center of the tank is
possibly too high for the inverted engine, a problem with a lot of the
ARF's now days the tanks are mounted wrongly for inverted engine
installations, the tank center needs to be no higher than the center of
the spraybar of the carb, otherwise fuel will siphon out through the carb
and you won't get a reliable idle or will find the top end also hard to keep set.
Check the tank would be my first suggeation to try and cure the problem
you have stated having.

Bill D.