noob needs help with mileage - only 10 MPG!

Very first thing is a general tune-up for diagnostics purposes

Does this still have adjustable lifters, that is, mechanical? (Not familiar with newer slants)

If so adjust the valves

Run a compression/ leak down check

Check the timing, and I don't just mean initial, or "idle" timing

Check that the mechanical advance advances, and that the vacuum advance works, and check how much advance you are actually getting.

Hold the throttle at an RPM to simulate "highway cruise" and listen and look down the carb throat. If you hear a "popping" or can look down the throat and see gas dripping into the carb, you have a problem, float level too high, etc.

Also, what exactly are you using to check MPG? Speedo? You sure it's correct? You have a GPS? Use that instead.