I did bring up the topic of leaning out the mix with him.  If i remember correctly, my instructor said that running too lean will shorten the life of the engine and it was good to run with a "slightly rich" mix.  He did a basic test where the throttle response was good after idling a while, then he lifted the plane and pointed it up and down and all around and it did not stall.  Apparently if the engine was too rich throttle response would have been slow.  If it was too lean the plane would have stalled when he pointed it all over. He does have the stock tank, it was the first thing i asked him. 

As far as i can tell there is no leak, but the tank in the nexstar was factory installed, and getting it out looks like it involves unmounting the engine, something i must admit i am a little fearful to do.  So hopefully nothing is wrong with the clunk inside the tank or something like that.</p>