Backfire through the carb? Almost sounds like the timing could be off 180 degrees.
Double check your timing, making sure the rotor is pointing to #1 plug on the dist cap when #1 cylinder is on the compression stroke.
re. your carb. These engines will start and run with the power valve stuck in either position, so the power valve problem may not be your starting problem. Make sure your idle mixture is turned out 1.5 turns and don't be afraid to crank the idle speed screw down enough to get a fast idle. You can adjust it down after it's running. Make sure you see a good squirt of fuel in the venturi when you manually open the throttle to max open. If you do the carb should be setup to atleast start it up.
If your rings are new and the valves fully close and were renewed you should have good enough compression to start.
Double check your timing and make sure the plug wires are in the cap correctly.
Good luck
Dave
