Sounds like the motor's been sitting a while. Idles good and wont rev up with any power. My experience with that is that your carb needs to be disassembled and the jets/passages thoroughly cleared from the gunk that old gas leaves in there. Just flushing with carb cleaner doesn't cut it. Use some fine copper wire to run through the jets. You can boil the carb body in a pot of water to clear out the small passages. Boil it a while! Replace any gaskets you tear up.
That's what I would do.