How can I fix a 1987 Mazda pickup that won't idle right?

First, I would do a good tune up. This will consist of new plugs, wires, cap/rotor button, new oxygen sensor (if very old) and clean the EGR valve below the carburetor. In short to clean the EGR, remove it has 3 bolts 10 mm and one vacuum line, one electrical connector. When removing NOTE the metal gasket it has position and clean it with carburetor cleaner and sandpaper or a wire brush or toothbrush to remove the carbon build up. Also remove the air cleaner housing the whole part (or just lift up enough and prop it with a screwdriver) and clean the carburetor linkage with carb cleaner and (I use brake cleaner) on the linkage so it works better and then lubricate the linkage with liquid wrench on the choke and cables. I would also replace the air filter, PCV valve in the valve cover. It is not necessary but it is a good idea to change the oil and filter to clear the fuel out of the oil if it is running rich. Also change the fuel filter. This should clear up the problem unless the air to fuel valve solenoid on the carburetor is going bad or worn. I could list more items but, start with a basic cleaning and tune job. I have owned my 87 b2200 for 23 years I know this truck.