https://modelenginenews.org/techniques/minid.html Mentions that this engine needs a spring start and 50% ether and it runs up to 40k rpm(!) seems like it’s finicky, which makes sense given its size

The spring start makes a lot of sense, thanks.

It was a mixture of ether, castor oil, and something else. He theorised that the ether was boiling off in the tank before it made it to the engine.

Ah thanks - I was imagining that perhaps the engine didn’t have high enough compression to achieve ignition.

I would think there would be some interaction between the carburetor air intake and the propeller right next to it. Intentional feedback, maybe, to change the mixture as the RPM goes up? If not intentional, then it sounds like a potential showstopper.

He did: It was just warm enough for the ether in the mix to evaporate long before it reached the engine, so it couldn't run.