Fred,
If the fuel rack is properly adjusted (the racks are nearly fool proof), look at the tappet, if it's operating properly, begin working your way up (air in the line, bad deliverly valve, bad element, leak, injector spring (very rare to die), nozzle itself... If you can't find it in the fuel system something internal most likely failed... howver not really probable. It could be as easy as a valve lash adjustment or the push rod bent... it'll most likely be something incredibly simple.
Bill