RPM AND TROUBLE SHOOTING

In jets, the engine RPM at wide open throttle (WOT) is essentially unaffected by load, hull condition or boat speed. If the tachometer is correct, changes in RPM at WOT indicate problems with the jet pump or the engine. Lower RPM indicates an engine that has lost power. As a rule of thumb every drop of 100 RPM indicates a loss of 20 to 25 horsepower at WOT. Higher RPM might mean those engine modifications you made are paying off. If no pump or impeller changes were made, every gain of 100 RPM at WOT indicates a gain of 20 to 25 horsepower. However if engine power has not increased, Higher RPM indicates a problem with the jet. If the RPM's have increased slowly over time, silt and sand may have opend the impeller/wear ring clearance excessively. Or the impeller vanes may have become dull and nicked, causing cavitation. If the higher RPMs and loss of thrust or "slipping" happened suddenly, check for foreign objects. Weeds, rocks, plastic bags, ropes, etc. stuck in the intake grate or wrapped around the shaft will cause cavitation and "slipping". Small rocks or sticks lodged in the impeller will do the same and also set up a severe vibration, as will a broken impeller vane. If the slipping only occurs on take off then air may be leaking into the suction piece. The hand hole cover cover may be loose or its O-ring damaged. The packing may be worn out or the packing gland may need adjustment. Air can also leak past the transom if there is a channel along a ride plate, skid plate or "shoe" for it to follow. Once the boat is on plane, the suction piece has positive pressure and no longer sucks air through the hand hole or packing.