88' 2.6L Mighty Max 4x4 Running Rich

I am looking for help and understanding with my truck. I drove it from FL to TX and it ran rich....extremely rich. 100 miles to a tank rich. I bought a carb (Weber man trans, electric choke) but have not installed it yet. I replaced the O2 Sensor just below the exhaust heat shield and it seemed to run a bit lean for about 200 miles then started running rich again, giving it a misfire feeling. I have recently bought another O2 sensor to install just in case the other one failed. I am not sure if it is a carb problem or an O2 sensor issue.

Question 1. Why does a carb'd truck have an O2 sensor? Is it for the electric choke? The electric choke has never worked since I have owned the truck.

Question 2. the tube that runs from the exhaust shield to the intake of the carb needs to be hooked up but why? Would that change the way that the carburetor functions?

I have just returned from Germany and my truck has set for 4 years with one of my parents driving/starting it every week. I have replaced the plugs, wires, distributor cap, brakes, brake master cylinder, O2 sensor.

Any help is greatly appreciated. I can be emailed at mustangcobra1789@yahoo.com. Thank you!

In the 80's they started using "feed back" Carbs and started putting throttle position sensors on them. It was all about emission's, so not only do you have a carb you also have a ECU. Best thing to do is replace the carb with a real Weber or find a pre-85 carb. then strip out the ecu and the sensors it hooked up to. One of the worst cars for this "problem" was an 85-86 Mustang 2.3L, the carb base would loosen off and the thing would run pig rich as the computer saw a vacuum leak.