I’m going to make some assumptions, WHICH I KNOW I SHOULDN’T DO, as it can get me in trouble very quickly. Here they are. Your surging is the result of a lean running condition, not rich. Closing the choke slightly helps a little. Carb has been cleaned and rebuilt with new float needle and seat. New manifold gasket and you have tested for leakage. Internal filter screen is clean. Using fresh fuel. Three things that I’ve found when working on these engines. 1. Float level not set properly, I normally set them slightly rich. 2. Mixture screw not adjusted properly, again I normally se it slightly rich. 3. Governor not set to correct speed, I like my engines set at 3200 RPMs.