I just bought one of these engines used. Air bubbles were appearing after the fuel passed through the needle valve. I cut a short piece of fuel tubing (about 3/32"" long) and put it onto the needle valve so it would be squeezed between the needle valve and the needle valve housing when the needle is screwed in to the correct location. It sealed this area and the bubbles stopped. The clicker spring has a bend in it that passes over the fuel tubing, so the clicker still works. The fuel tubing also keeps the needle valve from vibrating and loosening up its threads.

This is an easy fix and only takes a couple of minutes. Put the fuel tubing over the end of a needle nose pliers to stretch it open. Then slide the needle into it and pull out the pliers.