750 is high for most applications. the screw is the only idle adjustment.
make sure there is no power to the high idle electrical wire - the one that connects to two locations on the pump.
Also check inlet pump fuel pressure, high pressure advances timing and the electrical raises rpm for cold start up.
put Clear tube in place of the rubber one on the return fuel line to ensure no air is in diesel fuel. it is about 5" long and 1/4"
X2 for what @Will L. said. Air or lack of fuel will have the pumps do weird stuff. Like Knock, surge, etc. You know the sound of a 6.2 that is about to run out of fuel right?
Cruise control cable holding it open? Bent bracket? Can you manually close the throttle? Is the high idle solenoid locked up?
Remove everything attached to the throttle but the return springs to see if it still has 750 RPM. This includes the TPS or Vac switch on the passenger side of the pump. The passenger side of the pump stuff can be so far out of adjustment it holds the throttle open. Or switch or TPS is binding...
Is your RPM tool is accurate? Using the dash tachometer with the wrong "gas engine" pulley on the alternator will screw the RPM reading up.