A surging, high, or uneven idle is one of the classic signs of a vacuum leak. Check the vacuum line going from the intake to the carb or petcock to make sure both ends are connected and sealing properly, and try pulling it off entirely to check the line for cracks or splits.

Another common issue I have had is the spacer between the carb and the cylinder head leaking, this one is difficult to diagnose but if you've eliminated all other sources of vacuum leaks that's a good place to look. I've had to pull the spacer off of the head and replace it and reseal both ends to eliminate the leak before (actually just did it again last weekend). You can use a paper gasket and whatever sealant you have on hand, although I recommend the Permatex 85420 or equivalent since it won't dry up and cause leaks again like the other RTV style liquid gasket makers.

https://www.amazon.com/Permatex-85420-Permashield-Resistant-Dressing/dp/B007VIGCJW