Idunno.

All other possible sources for airleaks isolated, and confirmed fuel system integrity...... almost sounds like a crack or pinhole in the case, head, carb or something. Thermal related... lets air in after the engine runs and warms up.

I am a bit confused though. Perhaps I was not descriptive enough in my question. Just trying to isolate cause/effect.

Does the RPM rise happen within 30-60 seconds from when you first take it to stable full throttle (rapid rate) ?
In other words, it runs at 12K rpm for a minute, then over the next minute begins to lean to 14K ?

Does it quickly jump the 2000 rpm at a specific tank level?

Or does it creep up there in rpm as the tank level drops ?

Perhaps best answered following a subsequent test run.

Bob