I’m having exact same problem. When Start from cold and dark, the RPM barely goes over 2100-2300. if restart with the plane and the end of runway (already started) and the RPM reach 2500rpm. I tested the magnetos, mixture etc but didn’t find correlation. for information, It look like to occurs on cold temperature. It behave like a foul sparkplug. I got a foul parkplug on a real C150 and was behave like this.