1990 Ford Bronco bad gas mileage

There could be alot. I know you said you adjusted the timing. The computer usually automatically does this. So there would be no need to adjust it manually. Try replacing the timing chain. If its too loose it could jump time or not advance like its supposed to. If you haven't yet, replace the distributor cap and rotor, plugs, wires, and air filter. Make sure there's no drag, like grabbing brakes, and etc.

I have the same issue and with some checking the fuel injector rail pressure was 3 times the remmended level. The return line is either blocked or the regulator (on the fuel rail) is not properly functioning so I am told. I plan to replace both to eliminate either scenario and hope it works.