Okay, not nearly enough information but:
The 1978 280Z has an electronic ignition module inside the cabin - under the glove box area. The Blue wire from that ignition module serves two purposes - it sends the signal to fire the coil AND it provides the Tach signal. That ignition module is not used when you do an L28ET swap (which is what i am ASSUMING you have) The L28ET uses a completely different electronic ignition module controlled by the ECCS ECU.
You should completely remove the stock electronic ignition module if you are actually firing the coil some other way. THAT HOWEVER has absolutely nothing to do with the fueling of an L28ET... But then, I have no idea if you actually have an L28ET swap because you just say that you have "a 81zx turbo harness"... So...
(I WILL say: The 280Z Tach works just fine with a correctly installed and wired stock L28ET using the stock 280ZXT Coil and Ignitor when the 280Z electronic ignition module is removed and that blue wire is connected to the "-" post of the turbo coil (plus many other coil/ignitor configurations - I've done a bunch of different things to spark an L28ET or other L-series engine))
