I have a 2002 Scag 61" Turf Tiger with 25hp Kohler motor. 685 Hours on it. All of the sudden when you start the mower, it fires right up then shuts off. i crank it again, it fires right up, then stuts down. I changed the fuel filter....loaded with debris. Fired right up and ran OK, Engaged blades, shut off. Tried again, Fired right up...engaging blades shuts it down. Looked at new fuel filter, looked pretty dirty already. So I drained tank completely. Blew out lines and put new fuel in and new fuel filter. Oulled carb off and cleaned it out. Once again, it fired right up...but runs good for a minute or two then tries to die out...then revs back up for a minute or two, then tries to die. Engaging blades kills it still. I am starting to think fuel pump, but I am not a mechanic. I try to do my own work to save $$$ Any suggestions?? If it is fuel pump, are they expensive ??? And are they hard to change??