Reverse gate not locked in forward – Engine RPM is below 4000 at full throttle

Our jet drives operate in the 5000-6000 RPM range at full throttle.  If the motor will only turn 3500-4000 RPM at full throttle, it could be the reverse cam gate is improperly adjusted and not locked in forward.  On the cam / roller equipped reverse gate linkage, the roller must be all the way to the bottom of the cam slot in forward to lock the gate.  During acceleration, if the gate is not locked in forward, wave action can push the gate toward reverse, which reduces forward thrust and can cause an exhaust back pressure which reduces RPM.  Another reason for low RPM at full throttle could be due to sensors detecting over heating or low oil conditions. Cam Gate Adjustment Sheet

Rattle sound at idle

A rattle sound at idle is usually caused by a loose impeller nut allowing rotational looseness of the impeller on the driveshaft. This can be confirmed if when increasing the RPM, which loads the impeller, the rattle goes away.

Cavitation burns on impeller

On motors equipped with power trim, trimming out beyond a vertical driveshaft position reduces the ram angle of the water intake. The reduced ram pressure on the water entering the pump can cause cavitation burns on the impeller. Blunted leading edges on the impeller blades caused by rock damage can contribute to cavitation burns in addition to slippage on full throttle starts.

Engine over speed and loss of thrust

Occasionally when either holding or proceeding slowly upstream in shallow water, when the throttle is opened, the motor over speeds with no increase in thrust. This is usually caused by suction holding rocks against the intake grill, restricting the water flow. If you will quickly stop the motor and restart it, and rocks are not jammed in the grill, they will usually fall away and full power will be restored.

Driveshaft wobble in bearing housing before assembly

Before the driveshaft assembly is mounted in the motor, the driveshaft will appear to wobble in the bearings. This is a normal condition. When mounted in the motor, the upper end of the driveshaft is supported in the crankshaft and the system becomes quite rigid under load. If you are concerned about the condition of the bearings, watch the texture of the grease coming out of the lube hose during lubrication. The 630-AA Lubriplate grease coming out will normally be light gray. If water is present, the seals need to be replaced. If the grease starts turning dark gray, the bearing should be inspected and replaced if necessary.