I would agree with this 100%. Pull the tank and check your lines, both soft and the metal sections in the tank stopper, carefully from the clunk to the carb. I had a similar issue, also with a 40FP, a couple of years back. Turned out to be a small crack in the brass tube through the stopper. It would run fine until the fuel level was below the tube in the tank, then draw air, lean out and die. Drove us nuts trying to track it down. Good luck.