Impeller pitch and diameter is based on sea level operation. At higher elevation, you will lose RPM and may have to drop down one impeller size. Long use in corrosive conditions may create pin holes in the exhaust tube which builds up back pressure and reduces RPM. Excessive RPM is usually caused by air entering the pump. It can be caused also, during start up, if the leading edges of the blades are blunted from gravel. A rough boat bottom, keels or damage ahead of the intake can introduce air, as well as setting the motor too high.

Posted in: Maintenance & Ownership