What can be the possible cause of lost of power when the engine is running at 2000 RPM?

most engines have a power band witch is different most engines build power from 3,000 RPM to 8,000 but really it could be that your manifold is not set right or you don't have the carb tuned to the engine some other problems is that your valves are sticking but the answer on the rpm is just how big of engine u have and if all your parts match.