My Gravely 34Z has a Kawasaki 15HP, FH451V engine. After around 20 minutes of operation it acts like it is not getting enough gas. Chocking it helps but only for a few seconds. It stalls. Changing from one fuel tank to the other does not help. Leaving the gas tank caps loose does not help. If I wait until it cools, around 15 minutes or so, it will start and run for another 15 or 20 minutes before stalling again. Being mechanically challenged, the only thing I could think to do was replace the fuel filter. It did not help. Any ideas what is going on?