There have been some threads about using Armorall to prevent bubbles from forming in fuel tanks. In describing how it works, they talk about "excitation points" or miniscule sharp projections as being the initiation points for bubble formation. Somehow, Armorall does something (coats these points to remove the sharpness, lowers fuel viscosity, or ?) to stop the bubbles from forming.

This could be what is happening inside the needle valve body. Someone needs to try some Armorall to see if it solves this problem with the 91 FX. It only takes a few drops of Armorall per gallon. Robotech's $200 paperweight would be a good experiment, since it seems to have a severe case of the bubbles. How about it Robotech?

The other thought would be to remove sharp corners from inside the needle valve assembly by polishing. This could explain why some engines have the bubble problem and others don't.