RE: OS 55AX
Generally, if an engine behaves differently in different positions (upright/inverted), it is a fuel delivery system problem (tank and fuel lines/vent line), NOT an engine problem. In this instance, I am talking about an engine that starts well and runs well until the model's position is changed by roughly 180 degrees. It's early and I'm working on my first cup of coffee, so please forgive the awkwardness in my writing.
Good running glow engines do not have a preference in positioning once running well. So, the original cause of the engine's problems could be the tank position in the model. Something that would cause a change in engine running from previous flights to the last would be if the previous landing was a bit rougher than usual, which caused the clunk in the fuel tank to slide forward near the neck of the tank. The engine would start and tune normally and would probably fly pretty well for a minute or two and then there would be symptoms displaying of a fuel delivery problem. Rolling to inverted flight would eventually cause the engine to quit, with previous symptoms of running too lean at times up to that point.
It saddens me to see that OS is not using a bushing in the lower connecting rod end. Bushings were there to prevent exactly what happened to your engine. Just having a high silicon aluminum alloy in the the connecting rod is insufficient. A bronze bushing would have, due to its porous nature, most likely made it through these lean runs with ease, because of oil being stored in the bushing's pores. I wish OS would pull their heads from hidden caves and begin using bushings again. That is the mark of a high quality engine.
Ed Cregger