I was getting pretty horrible milage in mine as well, and a roughness in the engine, I replace the ignition control module and coil pack, and all is better now. While your at that, you may as well get new plugs/wires, as they can boost as well and don't cost too much.
I replaced just the plugs and wires in my 89 chevy G20 van (5.7L V8) and it increased the milage from 11-12 to 16-17mpg.
-Greg C.