My car is getting shitty mileage, 17 mpg city, 24 highway, so I did basic tune up. Well, more than basic;

changed plugs, set gaps to 1.1
changed wires
changed fuel filter
changed air filter
changed pcv
changed o2 sensor
changed distributor cap

Now, im still getting the same shitty mileage. My two last resorts are retarding timing (engine pings with regular fuel) and replacing distributor rotor. I can't figure out how to remove the old rotor and have no idea how to adjust timing, any of you wanna give me some instructions?