S14 SR20DET Running Rich

[TrisOCE]

Last year I picked up an S13 silvia with an S14 sr20det in it. Have been fixing it up slowly for a while, few things such as misfires, iacv and turbo issues.
It seems I am coming to the end, and it should be on the road quite soon.

I am currently chasing an issue, I thought it was injectors but I have swapped them out and it doesn't seem to have improved. Car is running rich (on all 4, every plug indicates so).
I get some puffs of black smoke when revving it, nothing too absurd, with a little bit of a stumble when revving it hard. After around 10-15 minutes of running it (idle with some revving), the oil smells like fuel. I dropped the oil and it's substantially thinner than it was, definitely has a decent chunk of fuel in it. Plugs are fouled within a few minutes of running it.
I just can't seem to find the cause of it. The loom is a little iffy since the last owner swapped the motor in, will get around to doing it properly eventually but for now I just want the thing registered.

I currently only have one O2 sensor installed, as I have no bung for the secondary one in the cat. I'm running an S14A ecu iirc, and I'm pretty sure all the important stuff is connected. 3" exhaust through the thing, but aside from that its pretty stock.
Would the secondary O2 sensor cause it to run so rich? I think I read somewhere that the first O2 is the only one related to engine management and the second is more so for CEL. I will replace the first O2 sensor just to make sure, but I can't find any other issues that would cause it to run so rich.
I'm just lost at this stage so any help is appreciated. It's the last thing it needs to get on the road.

Update: I have ordered an O2 sensor, will be replacing the FPR, cleaning the plugs again. Any other ideas? Or is it just worth buying a nistune and tuning it out?

[VStar650CL]

Have you checked the ECT sensor? The dash gauge and the ECM have separate sensors, so what you see in the cluster and what the ECM sees are two different things. If the ECT is reading colder than actual, it will make the fuel system run very rich.

[TrisOCE]

Have you checked the ECT sensor? The dash gauge and the ECM have separate sensors, so what you see in the cluster and what the ECM sees are two different things. If the ECT is reading colder than actual, it will make the fuel system run very rich.

The plug was broken when I got the car, this may actually be the issue. I put a new plug on it but maybe I got the wires the wrong way around (I can't imagine this would change anything, as I assume it just varies the resistance based on temperature).
I should however note, the car has barely had time to warm up in the few times I've ran it. So I'm not sure if this is the issue or not, but I guess I'll see if that fixes anything.

I'll order a new plug and fix that up.
I also have a new O2 sensor on the way as it looked pretty cooked.

[TrisOCE]

Update:

Currently have red injectors in it. I got a stock set of S14 injectors on marketplace with a rail. I assume that theyre 370cc as theyre blue (I think jecs OEM replacement).
However the car ran even more rich when I put these in. Could I have the 270cc injectors in it? Its an S14A ECU (so I assume the engine is S14A too).
Bit lost with the whole injectors side of things.
Just want the engine running in a mostly stock form before I bother upgrading injectors etc.