When you are finished with the standard or normal things. Close off the return line from the injection pump and report any change on this site.

No change at all means you have really low fuel supply pressure in the base of the injection pump. . You cannot hurt the car with this test.

If car goes much better the relief valve spring is possibly very weak. At that point you should get a thirty pound liquid filled gauge and check the actual pressure. Your ideal target is 19 pounds but around fifteen is okay with the return line open and not the major issue. With the return line obstructed and a lift pump in good shape the gauge should go to the peg at thirty pounds.

In my opinion there are many of these cars running with very little base fuel pressure in the injection pump.