If your pump is producing excess horsepower, it is time for a cleaning and maintenance run. You should start by looking at the most obvious place - the fuel tank. Fuel particles can build up there and create blockages, so it is important to clean it out regularly to improve performance. Use a small brush or vacuum to clean out the fuel tank.
The next step is to inspect your air intake and carburetor. Are the intakes contaminated with sludge or debris from the combustion process? Are they clogged with dirt or burned material from the fuel? A good way to tell is to open up both intakes and observe if you can see small brown stains on the carburetor body - these are air pollutants from the combustion process. If you notice them, then remove them and clean the intakes with a vacuum cleaner.
Next, look for cavitation bubbles. Bubble cavitation is caused by low pressure and high temperatures, two things that happen in most domestic engines. Smaller cavitation pumps use a similar principle as the Centrifugal Pump - essentially pumping a large amount of liquid (amines, oil, or coolant) into a very small area, which causes a very high rise in pressure.
Open your drainpipe and look for bubbles at the bottom. Bubbles in drains are usually caused by the pump not being primed correctly. See fig 6 for some explanation. When the pump is primed it draws the liquid from the reservoir backwards through a passage called a suction side tube, which has a small hole in it so that the liquid is drawn into the pump with a downward pressure gradient.
Next, check the rotational energy of your pump. Rotary pumps use a centrifugal pump, which moves the liquid using a shaft and a magnetic field. The rotational energy is the product of the rotation and the speed of the pump, measured in rotating seconds. Centrifugal pumps can also have a fan incorporated into their design, which works together with the centrifugal pump to increase the overall rotational energy of the device.
Finally, self-primed pumps tend to leak a little fluid as they run, due to the lack of an evacuated tube. To reduce this leakage, you might want to look at models that have an evacuated interior. Some self-prime pumps are also designed to be self-parasitic - that is, they only produce the amount of fluid that is needed to fill the container, so you don't need to pump any extra fluid when you change the volume of the container.