i replaced the entire back plate which included a new needle valve and seals, i ran the motor and had no bubbles between the needle and carb for about two minutes , then they came back when the engine warmed up. I gave up and bought a new os 75 ax because the needle is in the carb and it has no backplate to transfer heat to the fuel .I wanted a little more power for my plane anyway , its almost like the fx motor vapor locks.thank you for the info.