Very common problem with Z31s! Fuel gauge shows empty at 1/2 tank. Delt with this problem 37 years until retirement. Just found the reason for failure!
Most owners say to replace the old unit with $400 new unit. I fixed mine with a 35-cent resistor!
I own an 87na with analog dash. Tore apart the factory cylindrical sender (black wiring sheath) to find out it's the wrong one. The main tank resistive strip ohmed out from 0 to 520 ohms. 0 being full and 520 at empty. FSM says for an analog gauge, the ohms between pins 1 and 2 should be <6 ohms at full and >80 at empty. With the wrong strip, I hit 80 ohms at 1/2 tank! Solution was to place a 68 ohm resistor across pins 1 and 2. This parallel resistance approached the FSM specs and
the gauge works as intended!
The 1/4 tank gauge can be made usable again with another 68 ohm resistor across pins 1 and 3. This places the red needle at full until the low fuel bulb lights and the tank approaches empty. Due to the bad strip, the 1/4 tank gauge won't start dropping until the low fuel bulb lights. In any case, it's time to fill up!