so you are saying that it won't fire up now that you had the carb rebuilt. who re and re'd the carb , you or them?. theoretically the carb should be better now than it was before,so if nothing else was changed or altered it should fire up. unless you can tell us if anything else transpiered it is hard to say.
reason for backiring is usually that it is firing on open valves, perhaps in your case burnt valves or the ignition is doing something to make that happen
things to do and check
1 is the carb back on properly, not sucking air at the carb base-gskt problem
2 give it a compression test as that will give you an idication of what the valves are about
3 check out your distr that the points are set and working properly. check the wires inside the distr are all tight and n perhaps the wire itself is not pulling away from the wire connector end(where it crimps on the wire) as sometimes when you step on the throttle thee ignition advances and pulls that wire out and then relaxes back into that position when you let your foot off the throttle and make connection again
ron
