I do not know about the Yanmar
engine--but I once had an engine with a similar oil fumes condenser-recovery system that was allowing oil into the inlet manifold. There was a series of metal plates designed to condense oil vapour and to allow it to drip back into the oil gallery and hence back to the sump. They become carbon coated and not comparatively cold enough to condense the oil, which is then drawn into the inlet manifold. Cleaning
the plates only provided a temporary sort of solution, but for me it was just something one had to do. It was a bad design in the first place. In a few months I would have to clean it all again. It was not a gauze though--a pack of metal plates with a thin spacer between each so that the oil-air mixture could pass between them--and this is what blocks up with carbon and sludge.
Wire brush and petrol or kerosene shifts it. If you have one using a coarse wire wool system, the replacement one can try is soap-less stainless steel
kitchen scouring pads. Cheap
as chips at any supermarket. Throw away when dirty and replace with more of the same, but be sure they can not be sucked into the engine. They HAVE to have a stainless steel
gauze baffle to prevent this. Mine came from a camping toaster.