The thing i'm confused about, is why does a carb have an ECU and how does it work? Is it for timing/electric choke?
I only have the FSM for 84, so some things will have changed by 90.
In 84 Calif models, the computer primarily controlled the air injection system via the O2 sensor (Called Air Injection with feedback), sending air into either the exhaust stream or back to the air cleaner to keep the O2 levels optimum for the 3-way cat. The O2 sensor was in front of the cat (manifold, I think), & there was a *temperature probe* IN the cat. So the O2 sensor had no effect on the mixture *into* the engine, only on the amount of O2 fed to the TWC.
84 Federal & Canadian models used an "Air Suction system" without a smog pump or computer control. They also used a standard cat.
Both models appear to have some computer controls on a couple of other emission systems, like the Fuel deceleration cut solenoid,Cold Mixture heater, & evaporative emission control.
It's highly likely that by 90, many of the 84 Calif emission systems were in use in Canada & federal models, perhaps even a few other systems as well(Like timing,elect choke, EGR). Only the FSM for your model year will have all the correct details.