I have a grasshopper 721D that started stalling out this summer. It started where it would run for about 10-20 minutes before dying. Now it immediately shuts off when cranked. I'm not a mechanical person, I also inherited this lawnmower and would like to attempt fixing it myself to learn about it. I've recently changed the fuel filter and bled the fuel lines. Any tips are appreciated. Its also a diesel engine (wasn't sure if all 721D mowers were)