Air is getting in somehow. I'm not familure with the RNV set up on the OS, but I'm guessing there are at least 3 fittings screwed into one common point where the bubbles are starting (fuel-in, fuel-out, needle assembly). Remove these, clean up the threads good with solvent or alchohol, and reinstall them with red RTV to seal them up. If there is room on the needle assembly for a small piece of fuel tubing to seal it up do that...or if it is an o-ring needle set up, check to make sure the o-ring isn't broken or gone.