High rpm dont mean it's capable of pouring the power to it under load. When the load hits it the governor opens the throttle plate to keep the rpm constant but it has to have enough fuel to provide the power. Opening the throttle with no load on it will run the rpm up but it could still be lean.
Again, if you prove that it's rich enough then you move on. Maybe if I was standing there listening to it I would say buy the tone of the engine it seems to not be lean but I would still want to prove it. If it has a lean mixture it will have more of a tendency to hunt. If it's over rich it will tend to blubber. It's easier to notice it on gasoline but when adjusting the mixture or proportioning valve on propane you will still hear it.
