I have two Jett engines that use remote needle valves and never have bubbles in the fuel line between the remote needle and the carburetor. The trick is if you encounter bubbles between the needle valve and carb is to isolate the needle valve - remove it from the engine and run the engine again. If no bubbles, then you have a vibration issue causing the nucleation effect. Balancing the prop better should help, however mounting the needle valve on a firewall or anywhere else that's still inline with the carburetor should help and minimize bubbling fuel.