[datsun62]

Two of the main causes of running rich on a 280z are fuel pressure too high or a bad conection on the fuel mixture sensor
Fuel pressure should be around 30 psi engine running, when the fuel pressure regulator goes bad your fuel pressure normaly goes up and causes your engine to run rich.
There are 3 sensors on the front of the thermostat housing below the radiator hose on the front of the engine.
one is just a small nipple with a single wire it controls the tempeture guage.
The other two sensors look simular one is taller than the other,with a injector
type electrical conector.
The taller one controls the cold start injector.
The shorter one controls basic fuel mixture.
It is variable restior,when the car is cold the restance is high making the engine run richer, as the engine warms up the restance goes down causing
the engine to lean out and run normaly.
What happens to make it run rich is you get green corrosion on the conector and sensor giving it a bad conection or no conection at all and makes the engine run rich.
With a bad conection the computer senses high restance from the sensor
and adjusts the mixture too rich, this is the first thing I would check.
you can try to clean it up,the best thing to do is to replace the sensor and
the conector, you should be able to get both at most auto part stores.
look in your book for the proper sensor name or bring it to them so they can
match it up with the new one.
Just a word of warning don't do this when the engine is hot.