If the engine is too rich, then surging is normal. Also, you mentioned that you are still running them in. After you are able to get the engine to run through a tank of fuel at full throttle with the high-speed needle leaned to just rich of peak RPM, and you have found the correct idle mixture and RPM, then a reason surging would have to be tracked-down. Usually, it's a fuel delivery problem...bad muffler pressure, cracked fuel line, dirt and other trash in the carb, air getting into the fuel via vibration induced fuel moving around, a bad fuel tank position, badly-routed fuel lines, and so on. If you have trouble tracking down the problem in the model, then put the engine on a test stand and see if you still have operating difficulties. A test stand makes finding problems much easier than trying to do it on the model.