I pulled the tank and it was fine. I decided to replace the line to the clunk to just regular tubing. If anything, the rigid portion was restricting flow due to smaller ID.

I pressure tested the tank/tubing and it was good.

I ran with full tank and it was fine on any throttle settings.

At 1/2 tank, it was fine at low throttle but cut out on high when pointing down. However, it cut out after a much longer period. Ran maybe 5 seconds, slowed, then got fast, then died. I pulled out a 51 size engine and it did the same thing but ran for 10 seconds. I guess the fuel usage explains that. The 51 runs at any tank level and any orientation in the sky so I think this ST90 should be OK too. Now time to maiden and find out....

(I have been doing much more checking as this was a used engine and I want to make sure)