[Kurt in NJ]

Could it be something as simple as not cleaning up the burr from the end of the fuel pipe from when it was cut to length ---debris inside the pipe can also create points for bubbles to propogate, and I suspect that the sharp burr left from cutting the tubing is near a perfect shape to use to take advantage of the surface tension that exist in the boundary layers of a bubble to inhibit it's movement.

The pet hamsters water bottle uses the slight restriction to keep water in the bottle--

I have had condenser lock, points itus, and wads of jelly slime, but have yet to have vapor bubble lock ---even though my manifold is cracked just above the fuel bowl and blows exhaust on the carb ( it mostly closes when it is hot)
I had reamed the ends of the pipes on my car years ago just because I thought it could be a restriction.