I would use the choke, or something to substitute and richen the mixture with the choke to see if that stopped the surging. That will tell you if it is running too lean. You can cut the fuel off and let it die from starvation to see if it is too rich. If the rpm gets better when the mixture changes then you will know it is a mixture issue. If the mixture does not improve then you will know that it is not the carb. Sometimes the placement of the spring can make it more or less sensitive to surges. Also you can hold the throttle steady by hand and see if it runs steady. That will also tell you if the carb is metering fuel properly, and the governor is at fault. I've seen engines run lean also because the fuel flow into the carb is poor due to the filter or filter sock being plugged.
