why does 1984 nissan pu get only 3 miles to the gallon and run rough with new fuel filter, air filter, cap, rotor, wires, egr valve, accelerator pump, spark plugs, oil change, and carb rebuild.

I know you said you rebuild the carburetor but spray a can of carburetor cleaner into carb it not only cleans the carb but also cleans the intake and intake and exhaust valves also add carburetor cleaner to gas tank. Use the whole can of carburetor cleaner.

It would be my guess that the carb. rebuild was unsuccessful.

They had some kind of strange thermostatic part that keeps the choke shut when it fails. in years prior to 85, look and see if yours opens after a little while running. "rebuilding" a carb means studying and testing every part. not just a jiffy kit of a few gaskets. If yours stays shut, see if you can see a gear alongside that is actuated when the element part warms. It could be similar to what I have seen them have happen. we replaced that item. Then repeated the spark plug replacement. Another cause can be internal leakage following "rebuild". depends if you had some other issue result with what you had to do, I suppose. Any shop can check what I am saying to help find the cause. and retrace your tune practices for good or bad. new parts can be no good when you buy them, especially chinese wannabes. First, does it smell fuelly ? that leads to the checking I suggest.