I have a '73 rs that turns over but will not catch. It is starting to backfire through the muffler and somettimes the carb. I've changed the wires and points and condenser. I have the points set around .17 thousandths.

What could be my problem

It sounds like a couple of things could be wrong. First, check the easy things like the coil, cap, rotor, firing order (18436572 for those of you who haven't embedded these #'s in your heads already). Next, I would check to see if the timing is off (timing chain slipped). Last, you may want to pull a valve cover off to see if you have a valvetrain problem like a couple of bad lifters, broken rockers or a flat cam.

Check to see if you have blown a power valve..