Here's the update on today's testing.

1) Replaced quick drain valve with 1/8" NPT plug and put clear line between flow sensor and drain point. Test flight showed air and bubbles in that clear line. Gravity should have filled up the line after a while but didn't.

2) Removed flow sensor, clear line from pump to drain point. After running engine for a while the line finally filled up with fuel. Test flight showed air in that piece of line, and the "pressure" drop and bubbles in line from pump.

The line from tank to selector valve looks a bit squeezed at a bend, possibly the tank moves slightly in climb attitude and squeezes line more.

Plan for tomorrow:
Replace line from tank to valve (inspect fuel tank pickup at the same time) and test.
If no joy, figure out a way to eliminate selector valve (I'm not too happy at the prospect of not having a fuel shutoff valve, not even for a bried test flight).

Finn