It backfires thru the exhaust
bwssr, those old Wings use a lost spark type ignition so fire the spark plugs on both compression & exhaust.. Any fuel remaining in the exhaust system or cylinder will fire off on the first spark if air is present. You might try starting it without opening the throttle as that should lessen the air present during starting.
You could also have a slightly leaking carb & allowing fuel to drip into one cylinder.. It can also pull fuel/air into a cylinder & exhaust system from a carb on any cylinder that stops with both the intake & exhaust valve open, the hot exhaust can use convection to keep drawing through the cylinder & carb.
Things to try--
Try turning the kill switch off, then start cranking the engine, once it is cranking flip the kill switch on.
Try different choke settings to see if a different setting will kill that pop.
Try starting with less or more throttle opening.
Try either using or not using the center stand when parked hot.
Verify all exhaust joints are sealed & there are no holes in the exhaust system anywhere.
Try a different type/grade of gasoline as maybe you are getting a hot soak boiling of the fuel in the carbs.