87 Pickup 22r running rich, erratic idle, carb adj not effective

I have an 87 pickup 4x4 with 22r, manual trans. My truck started running very poorly about a month age. It was missing quite badly. I changed the plugs with out any results. I then checked the plug wires and dist cap both had some corrosion. I changed the dist cap, rotor and plug wires. This initally seemed to fix the problem. I drove the truck and it idled fine and seemed to run ok. When I drove to work the next day my truck was idling poorly and running very rich, there is black markings on my driveway from the exhaust. The truck will blow some blak smoke when revved high. If I try adjusting the carb with the fuel mixture there is no effect. if I adjust the idle so the truck will idle when it is cold, the idle is extremely high when when the truck warms up. Also the fast idle is not working. I am only getting about 10 miles per gal and I consistently got about 19 mpg in town before this problem. I thought some of the mileage problem was the fuel pump because it was leaking intermittently. I replaced the fuel pump and this did not solve the poor mpg problem. Also when I turn the truck off it will diesel on some occasions. On one instance I removed the hose to the pcv valve and the engine idle increased. I thought that the problem was a pcv valve. But when I checked the pcv valve it was ok and the idle did not increase when I removed the hose at a different time. My truck has 246,000 miles and is getting worn but the compression was still with in specs last time I checked it, The middle two cylinders were a bit lower than the others but that has consistently been the case since I first checked the compression. What do you think the problem might be? I have thought of vaccum leaks problems with the emmissions systems, O2 sensor, carb problems. I don't know much about the emmissions system and don't quite know where to start trouble shooting.

Any sugesstions?
Thanks, Dan