Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Great job sleuthing that out Jan. I recall having a similar incident on an engine (don't recall which engine, but it is in my notes ... somewhere) that did not have a porthole sight gauge, and was seriously mystified too. In the end I did flush the boiler out repeatedly and did get some small grains of some kind of solids out, and the behavior of the engine improved noticeably at the next firing.