Engine stalls when power needed

[Casefan47]

I have a John Deere 757 z track lawn mower with a 25 hp Kawasaki engine. Everything runs fine until there is a need for a little more power, then the motor will bog down and sometimes even kill. Fuel filter is not filling completely full and is empty after the motor kills. Just replaced the filter and pump thinking that was the problem, but my problems still exist. Could be something getting too hot? Any suggestions? Thanks.