noob needs help with mileage - only 10 MPG!

My 69 Dart 225 got 21-22 mpg hwy regularly. Adding electronic ignition (Crane) didn't noticeably help. It always idled too lean. Most likely the problem is in your carb. Do you have the Carter BBD 2 bbl that was common in 1980? Those are fairly rugged. Most likely the float has sunk or the inlet has debris so the needle won't close off, both of which can make the bowl overflow into the throat. If you have any of the emissions stuff from 1980, that could make it run bad unless in tip-top shape. Make sure the choke opens fully when warm. Down the road, consider adding a rich/lean indicator, which requires welding an O2 bung in the exhaust. That is the only way to really know how the carb is functioning.

Also check your ignition. Do you have points or the electronic stuff from the 1980? I assume the mechanics checked the spark timing. Adding HEI ignition is not hard or expensive and would take care of any questions on spark.

Finally, valve timing could make it run real bad. If you have the hydraulic lifters of the 1980, no need to adjust valves, but do verify the lifters aren't gunked up. If the timing belt is loose, it could even skip a tooth. Read up on how to check that by rotating the engine fwd and back while watching the distributor rotor.