Bill Noble wrote:
timing - did you shear the key between flywheel and crank?
valves - do they seal - backfire must be either timing or valves
points - properly adjusted? on B&S they are under the flywheel
sounds like an air leak, are you sure manifolds, head gasket, etc are not
leaking?

Did you line up the dots on the cam gear with the dots on the crank
gear? I don't recall you asking about it...