The popping out of the exhaust sounds rather as if you have a leaky valve, Paul. Let's try to summarise what we have:
1. The engine surges or hunts with the speed control not being moved.
2. If you richen the mixture, either by partly closing the choke or obstructing the air bleed, it improves substantially.
3. Recently you've noticed some minor popping from the exhaust.
These symptoms are a bit disparate Paul. Have you done a compression check?
I suggest you do that before making any further changes in the carburetor.
