Onan diesel generator quits after a few minuthes

Not familiar with an Onan diesel 6.5KW. 7.5KW, sure.

When this happens, before restarting, go out next to the generator and LISTEN while someone else pushes the prime button (if you have one). You are listening to the fuel pump. If it starts out at higher speed (AIR) and then slows down (pumping fuel) check the fuel system for a leak allowing air into the system. Could be low fuel level in the tank, a crack in the hose, a leak at the fuel filter.