I dont know if this will help but it was offered to me as advice when I had the same problem.
Add an inline fuel filter, it did seem to lower the amount of bubbles in my fuel line but never really got rid of them. Another person said a few drops of armor all in the fuel acts like an anitfoam agent......have not tried this myself but am passing it along.

I had not flown R/C in a very long time and having just returned, I noticed some changes, I notice that the fuel line wants to "sqeeze" itself back off almost any nipples its attached to(after its wet with fuel)are the fuel fittings being made slightly smaller then 15 years ago, or the fuel line, or maybe the materials used in either are somehow effecting the fit once exposed to glow fuel? I have never had this much trouble with fuel line in my life before, then again, twist ties never came with fuel accessories before either.........never needed them.....you attached the fuel line and it stayed put until you removed it to refuel or whatever........hmmmmmmm.........makes ya think a moment doesnt it? I am betting that the air in the fuelline at least on my plane is a product of this and is being sucked in thru a fitting, somewhere, somehow. My tank compartment is foamed, as per instructions, all my planes are, but its the one OS engine that gives me the air/fuel line fits.
Maybe I will try a different brand of fuel line on my next build.