If its backfiring out the carb, it's usually a valve and/or timing issue.
If it's backfiring out the exhaust, it's usually carb or ignition related.
("usually" of course does not mean "always")
Since it runs consistently for 7 seconds, I'd (loosely) rule out valves and timing - if were either of those it should happen immediately)
Assuming it's in the exhaust, it means you're getting unburned fuel exhausted into the exhaust manifold. It may take a little time (7 seconds perhaps) for the exhaust manifold to get hot enough to ignite that unburned fuel.
As counterintuitive as it sounds, that can often happen from the fuel mixture being too lean to burn fully in the cylinder, but it can also happen if it's too rich.
Either way, it's definitely worth playing with the carb.
Another issue could be a cylinder missing, passing unburnt fuel into the exhaust. So check for a good spark on all cylinders, and clean up your points, etc.
Make sure your plugs are clean, and wires are good. Make sure everything is dry.
If still having trouble, adjust your valves and take a closer look at your ignition components.
Essentially, check everything an anything that has to do with getting a proper burn in each cylinder.
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