Help, my engine still wont start

I am still trying to figure out why my 1970 4280 CJ will not start. All it does is backfire out the carb, which causes the carb to catch on fire. This Engine was running good before running rough briefly and then not starting again.

I have now tried several things. I think that it is not fuel related. It will not start with a shot of starter fluid. The carb does fill with fuel from the pump. I have tried a known carb from another 428, which behaves the same way on my engine. I have tried a different coil. I have replaced the distributer with a new one with standard points - still the same! It has been getting spark to the plugs the whole time and the distributer does turn counter clockwise when I crank the engine. My next idea is to pull a valve cover and see how the valves behave when starting. I would like to do a compression test but on a 428 it is very difticult to do so. I did put in rew irridium plugs last year, but it requires removing the smog tubes. I am wondering if something went drastically wrong with the timing chain. I can only guess that the timing is drastically off.

Any help would be most appreciated.

Thanks for your thoughts. It was professionally rebuilt with new timing chain about 12 years ago. I'm sure it does not have plastic gears. It has seen very very few miles since then. It is original even including original pistons, except for a mild crowler cam, and rhodes hydrolic lifters.

Thanks D.Hearne, I had never heard your trick with rotating the distributer to make it spark. I understand exactly what you are saying and I will use this great tip. In my old distributer, I did have pertronix, which I will put in the new distriburer as well. I actually replaced the distributer and electronic points to see if either might be my problem. I only replace the distributer when the ignition timing pointer is at my timing mark and the roror is pointing at #1. It is seeming to me that my cam timing some how went off though.