It seems gas engines do that, even in cars. If you drive them slow all the time, they become slow and carbonated.
I even notice with my 4-stroke that I have to drive it fast sometimes. If I drive slow on it a whole lot, it doesn't have as much throttle response after a while, but if I go wide-open for a few good runs, it runs much better afterward, and is more responsive. I only got my engine in June, and I can see the carbon buildup on the piston top when I look in the sparkplug hole.
With ANY engine, I recommend running it at the full RPM range every so often over the course of its life, to get the engine use to spinning the higher RPM and to keep it running good.
