Pulled the engine out of the model, mounted in an a test stand, and fired it up. It runs without apparent problems. No sags. Stable idle. Good high end. So what have I screwed up on the installation? This engine is mounted upright in the round metal cowl of a Flair Legionnaire. There's a dummy engine but with two "missing cylinders" for air inflow and a wide area above and behind the cylinder head for air outflow. Tank is pretty much on the carb center line.

In the case of the other engine, it's mounted sideways (needle valve pointing up) in a Flair Puppeteer. Also behind a dummy with cutouts for inflow and lots of space for outflow. More to the point, why was I able to fly the model (the Puppeteer) without problems of any kind for almost 8 hours over the past two weeks without any problem until just now? I haven't changed anything on the model or the engine. It just stopped working.

BTW, I notice the the fuel foams pretty badly on the test stand but it doesn't seem to affect the engine at all.