It's likely the plug might cool down too much at idle which would mean raising the idle speed to keep it lit or fitting a 4 stroke plug might help. Almost all engines have far too large a carb for good fuel draw which is why muffler pressure is needed for anything other than level flight (or a test stand). If you wanted to fly with no muffler and the engine wants to die off lean with the nose up check then a work-around fix is to set the carb barrel to only fo maybe 3/4 open and retune the mix for that opening. This simulates having a smaller carb with better fuel draw.

Running with no muffler has no affect on the piston because this was the way all engines used to run before noise became an issue.