The air bubbles could be from a number of places. First thing I would check is the tank! Do you have a split at the stopper section would be most likely. Second, do you have a bad stopper? This is not very common, but occasionally you get one that allows a small bit of air through. Third, have you tried replacing the pick up line inside of the tank? Lastly, have you tried new line from tank to needle?

If the tank is okay, the first thing I would do would be to open the tank and replace the pickup fuel line. Second reseat the stopper and only after the stopper is seated fully,coat the front of the tank near the stopper and the stopper area with a silicone sealant of some sort. This may not end any leaks at the stopper section, but will go a long way to keeping them from being big problems. Then I would replace the fuel line from the stopper to the needle valve with a quality fuel line.

Once you have done with this the air bubbles should be either gone, or at least lessened.

Now a small, key word is SMALL, amount of air bubbles will not generally due much harm. It will generally make the engine run a little more lean, but with a rich needle setting you should be okay. The key thing is to check for a CONSISTENT engine run. You want to have an engine that will run without the worry of it stopping suddlenly all the time! As long as you can get the engine to run consistent and well, a SMALL amount of air bubbles won't hurt much.

Just my thoughts,

Reg