If your carburetor needs a rebuild, it could be leaking the fuel out into the intake as it sits. This would require some cranking to re-prime it. The raw fuel that leaked down into the intake might cause the backfire too.
To test for this, let it sit and get cold (overnight?) then remove the aircleaner and look down into the primaries while you open the throttle by hand. Do you then see the accelerator pump squirt fuel into the engine? If not, the fuel had leaked out.
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