RE: YS 120NC bubbles in fuel line
It may possibly need a re-build of the regulator, replacing plunger, spring and diaphragm. Make sure that the air bleed hole in the carb barrel is drilled to 1/8". If not, do it yourself, as it is not difficult. The hole is located in one of the blades of barrel, and as stock it is about 1/16". This will help the problem you have if it has not been done yet.
Do this before doing the other fixes., and nothing further may be required.
Good Luck!