Try this it wont cost a dime ,Remove the hose going to the brake srvo block off the end with a bung ,so no vacumm can get out or air in .Try it now ..I go along with not messing with the mixture this is something simple. The fuel pressure regulator can get blocked up with dirt and rust this is in the same area as the fuel filter... Take a fuel pressure reading also .Do come back and let us know ,easy things first .