Alex,

this is a common problem of all YS engine.

I have two YS91AC with the same problem what you had today.
fortunately, all of them are recovered to normal.

First of all, check all screws, the screws are loose when exit factory or after first flight. it cause the pressure inside the engine very unstable. escpecially the valva cover and air chamber. becareful in tighten all screws.

Second, check the codition of the gaskets (valva cover and air chamber), replace it if broken where will cause pressure leak.

Third, screw out the regulator and carefully clean inside. sometime the lubricant oil will stick around there.

Nelson