bubbles in the line between a remote needle and the carburetor can be caused by leakage in the needle assembly, but can also be caused by air in the fuel before it reaches the needle. The needle has a stagnation point and micro-bubbles can 'stick' there and form larger ones, which you see.

If you replace the needle assembly and still get bubbles, then you'll have to examine the fuel system to see if you have an air leak of some kind previous to the needle.

If you have high fuel pressure, then you would not get air leaking into the fuel system at the needle. If there's potential for a leak, fuel would exit the needle assembly instead of air being sucked in...due to fuel pressure.

We'd try a new needle assembly, first.

Your difficulty is not something that is at all common with the O.S. .50 SX engine.