The 15FE is really a pretty simple engine. At 2 1/2 turns out, I would doubt that you were running it too lean! I ran mine at 2 to 2 1/2 turns out and it performed well through 3 to 4 gallons of fuel before I sold it. One of the problems with the NMT is that at idle, it vibrates like crazy. If I let mine idle for too terribly long, the fuel would start to foam and I'd get air bubbles in the line. End result was that it would stall.

Another thing you may want to look for is an air leak in one of the lines or at the base of the carb. Also check to see if the gas tank lid is sealing properly. If it's not, the fuel system doesn't build up the necessary pressure required to make everything operate properly. Lastly, check to see what temp you're running at. Anything over 250 is too hot for the 15FE.