Put one line (with a few elbows) direct from tank to pumps.
1st flight test, no significant bubbles at high power setting after the pump.
However, after reducing power I noticed an air (or vapor?) pocket showing up in the vertical piece of tubing from drain fitting up to tank. (Drain valve still replaced with a plug.)
I don't understand why air pockets remain in a line. Shouldn't the air rise up into the tank?
3/8" ID lines (elbows 1/4" to 5/16" inside).

Put everything, except fuel flow sensor, back in place: selector valve, fuel filter, quick drain plug (with a new line from tank to selector valve). After fuelling and opening the valve, I noticed an air pocket on top of the fuel filter and in line above it. Turning on and off the pumps, I got the pocket up higher and higher in the line above the filter, but not completely gone.
2nd flight test flight showed a pretty solid fuel flow and pressure at full power. But after reducing power I saw bubbles in line from pump to carbs, as well as more air (or vapor?) in line above filter.

So today's tests are still inconclusive. Sure with less restriction by leaving out valve and filter I saw less bubbles, but I don't understand the air pockets. I guess I need to device a way to completely purge all air from the lines.
The selector valve is still suspect.

Wondering if the vent line is too small.
Plan for tomorrow is to put a vent in the fuel cap, pointed into the wind, like I originally had until I got tired of gas showers whenever I botched a roll or loop or otherwise got near 0 Gs.
Restrictions in vent line are less than 3/16" (1/4" line with elbow in it).
Might also try bypassing the selector valve again. (Pain in the butt because I have to drain the tank first.)

If/when I go to fuel injection I will need to take out the fuselage tank and put more holes in it.
My tentative plan is to:
1) Remove the flow sensor and the 4-way selector valve and replace it with a simple shut-off valve followed by a T to two high-pressure pumps followed by one or two filters?
2) Return point to tank should be what size and position?
1/4", 5/16"?
Bottom of tank?
Top of tank?
3) Use the two facet pumps to transfer from wings to fuselage tank, presumeably to top of tank to avoid a flow back to the wing tanks. (If I ever get around to fix the leaks in the wing tanks).

What size fuel line to and from high pressure pumps?

Any recommendation for a simple shut-off valve?

Finn