To check for an internal leak you need to get some gas into the crankcase, propane will work fine. Start the engine, open the gas valve on the propane tank feeding the gas either to the pvc opening on the valve cover or the oil fill on the other side. If there is an internal leak then the engine will smooth out when it has injested the propane. If this isn't it look at the idle circuit of the carb since you are only seeing the problem during idle.