Gas cap is not venting or something float around in tank an covering outlet. Running 15 minutes usely cap vent lossen cap see if go pass 15 minutes.

Check venting as mentioned and triple check that you have the correct fuel filter especially if you have a Kohler engine.

It has been posted that sometimes the JD application offers the wrong fuel filter and in the Home Maintenance Kits too.

An accumulation of crud in the gas tank usually rolls around in there for a while, then it forms into a plug, and eventually plugs up the tank outlet. And, the engine starves for fuel. So, while you are standing there, scratching yer head, the plug falls apart, the fuel flows again--and the process begins anew.
Another thing that happens--the little hole in the gas cap gets plugged, by a bug egg, or dirt, and the engine stalls. Not being able to see the gasoline level thru the opaque plastic tank, and the engine has quit after a few minutes of running, you remove the cap and peer in! This releases the air-lock that forms in the tank, and fuel trickles down to refill the carb bowl. So, you try it--it starts--and runs--until another air lock forms in the tank, and the engine stalls again. By this time, you are wondering if the gas has all run out, so off comes the cap-the air lock opens up--the engine starts and runs--the air lock forms again--on and on, until you poke a wire thru the air hole in the cap! BTDT! Rustyj