This males sense in that I always wondered how a clunk could address that issue...however, how is it that an almost empty tank (in a 40 sized engine) can still run at any angle?

Is this just magnified due to the larger engine? (ie sucking much more fuel so tubing empties)? I just would be surprised that it would empty so fast (2 seconds)

If that is the problem, could I add a second clunk (off a TEE) on very short tube so it is always at the bottom-front of the tank? Adding more tubing does not sound like a good fix as it might be feet worse to be able to run given the 2 second issue...

Agree that your test will prove one way or the other. I guess I should also try at lower throttle level too. More troubleshooting.