help annoying pop in exhaust

A leak at the header flange or a cracked pipe up near the head,will let fresh air into the header and cause any unburned fuel there to continue burniing. The expanding gases will hurry through the primary pipe and and pop when they hit the collector.
A problem with an exhaust valve can do that too.If its not sealing then during the intake stroke the piston will draw exhaust into the chamber along with a partial charge from the carb. This then is a hot, and probably lean mixture. then on the compression stroke, some of that charge may escape into the primary pipe, where it may ignite and burn, leading to a pop.As the rpm goes up there is less and less time for these things to happen, and the popping quits.
A compression test or a leakdown test,on a cold engine may not catch this. I would do a leakdown test on a fully warmed up engine.
Late ignition timing can do it too.If the fire isnt started early enough, the fuel charge may not have all finished burning when the exhaust valve opens.In this case though most all the cylinders will be popping.
An incorrectly set mixture screw, or a faulty low speed circuit could lead to misfires where the mixture could also ignite in the primary, but it would require oxygen to be there and that takes us back to the leaking header flange.
Before I would do anything I would drop that header and inspect the gasket. I dont suppose you are using one of those emission heads with the AIR ports, and the header gasket doesnt cover it?That would be an oops moment.