This actually seems lie one of two things.
First, it sounds like the valves are not closing completely which is allowing the compression to push back into the carburetor. This will cause the backfires.
Thre are two things that can cause the ignition timing to be off.
1. A flywheel key that is sheared or partially sheared will cause the ignition timing to be off.
2. An elevtronic igniton coil that has gone bas will also cause this problem. The ignition coil has a spark advance circuit inside it. If this circuit goes bad, then coil will fire at the wrong time.
Pretty much these are the reasons for a backfire and non starting.
Normally I would say that a dirty carburetor can also cause this, but you have already replaced the carb.....BUT if you DID NOT clean the gas tank and put in ENTIRELY NEW GAS, the fuel system could still be causing the problem.
Remember...... Water in the gas can cause this problem...and.....You can NEVER use gas that is over 30 days old!
Well that is about all I have for you at this time.VAVES.Ignition Coil,Bad gas or water in the gas....
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