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.

You can also install soft copper iwre "twist tie" hoze clamps to help ensure the hoses are sealed to the barbs. Or, Hobby shops that sell R/C car stuff will also have miniature spring wire hoze clamps at about $0.25 each.

On one remote needle, I had to make a copper twist tie hose clamp and tie the clamp to the needle body to keep the fuel line from sliding off... There was no barb on the connection.

If the engine is running fine, it's not air bubbles it's vaporized alcohal and it's generally not a problem.