Q: Its backfiring mostly while driving, loss of power, fuel consumption, motor doesnt idle smooth, P0300 code, no timing codes

All of a sudden my motor runs like crap. I bought this motor used with around 80,000 miles on it. Just started backfiring out of the driver side exhaust (dual exhaust). Does it at idle and a lot when put in Drive gear. I did an oil change, spark plugs, spark plug wires, tried to set timing but its off 20-30 degrees after setting it right. No matter how I set the distributer its always +20 or more degrees. Like first setting in the distributer its at +20 and plug in the cam position sensor on the back of the distributer it goes to +30 degrees. I'm in Colorado so it should be at least 7 degrees. If its running this way and timing is way off no matter what, what are all the cases it could be? Was told timing chain and/or injectors.

My car has 160920 miles.
My car has an automatic transmission.