It's still a possibility it can be too lean. Remember, turn the needle valve one click at a time. Also the engine can be lean on the low end. Try and richin up the low intake valve and see what happens. But make sure you only richin up a 1-3 clicks at a time, because that screw is very sensitive and can be hard to get adjusted properly. If you have already tried these things and is sure it isn't that, then it can possibly be the fuel tank itself. Because needle valves don't go bad that often unless it's constantly being abused and stripped. And that wouldn't cause bubbles in the fuel line. After trying this and nothing happen, replace the fuel tank. I personally prefer the ultra flex fuel tanks.

GoodLuck