Check your vacuum control hoses. If they are split or perished it will affect your fuel economy. Clean out the carburettor with a can of carb cleaner and run a fuel system cleaner additive to the fuel tank. If the jets aren't atomising fuel evenly it won't burn clean. If your ignition coil is 15 years old, replace it. A tired ignition coil will affect economy and HP. Don't use the factory engine tune, it is lousy for power. My method of engine tuning is simple enough and will improve engine performance. With the engine running, undo the distributor retaining nut, hold the throttle @ 2000 rpm and gently adjust the distributor between advance and retard until the engine rpm's increase by itself. Back the distributor off a little and lock the distributor in place.

*Exodo, with your spark plug gap that close you may not be getting enough ignition heat to burn fuel cleanly. Open your plug gap up to at least .85mm (.032in) with my engine tune method and you should notice some improvement.