OK, well audio quality can always fool me… I’m guessing the snapping I hear is the disconnected low speed needle linkage slapping against the lower pan… But, the engine is certainly coughing/spitting lean. It almost sounds like it is dropping a cylinder, but that might just be its reaction from the lean spit/cough, again audio quality can be deceiving.
Are you sure the carb butterfly is completely closed? If not, it will certainly react like this.
What was wrong with the original carburetor? What carburetor did you replace it with? Are there any obvious external crankcase leaks? You would probably see obvious spray when the engine coughs lean if there is an external leak.
Usually the 20hps have a carburetor with a smaller throat, sometimes the intake manifold is restricted also. Putting a carb with a larger throat on this engine might cause a fuel/air flow issue through a intake manifold with a smaller opening for the carburetor…
