1990 Mazda B2600 Question: runs very rich 5 mpg

I am surprised it will run if it only gets 5 miles per gallon. Check fuel pressure, check regulated fuel pressure. Spec for fuel pressure is 64 to 85 psi but regulated pressure is 28 to 37 psi @ 20 inches of manifold vacuum, and 38 to 46 with no vacuum at the fuel pressure regulator.
There is a company that independent auto repair shops "buy" their information from that offer a very reasonable service to do it yourselvers. You can get a one week subscription or year subscription very cheap. The information is year make and model specific repair procedures, service bulletins, component locations, wiring diagrams ect.... great quality information same as the professionals have ,much better than generic workshop manuals from parts stores, much cheaper than the factory manual try http://bit.ly/mitchell_repair_manuals_online
and get an online repair manual subscription.