The MPG does not seem particularly bad, anything in the 20's for average is about right. My '85 has slightly higher compression than your '84 and I'm averaging 22 MPG. I would think that changing out the sensor should help some. I would also suggest running a few tanks of fuel with an injector cleaner, like Chevrons Techron or Redline's cleaner and see if you get more out of the 2.3.