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Engine rpms jumping with no effect on speed

Z

zackclan

New member
My 65hp 1973 Evinrude seems like the rpms are too high for how much acceleration I'm getting, and the you can hear the rpms get higher at higher throttle but it almost feels like the clutch slipping on a car.

Not sure if the engine propeller is taking in air or maybe something wrong with my gearcase?the prop has seen better days and is not very smooth on the edges.

When I was out going full throttle the rpms seemed way too high for how fast I was going and eventually the overheat alarm sounded so I stopped and went home going slow.

One thing that may help diagnose would be that the only other time I took the boat out I had another person sitting in the back and I didn't notice this problem.

Thank you.
 
It's possible your prop hub is bad. The prop blades are bonded with a rubber sleeve to inner metal case that mates to the prop shaft. That rubber hardens and can break over time. When this happens the shaft spins inside the hub without spinning the prop blades. You can get get the prop repaired and a new hub pressed in but it's often cheaper to just buy a new prop.

The other possibility is you motor is setting too high on the transom. The cavitation plate should be about even with the bottom of the hull. If the motor is too high you can get turbulent water running over the prop blades creating cavitation and loss of thrust.

The water pump issue could be completely separate from either possibility. However, if you are getting turbulent water in the pump intake that can mix air bubbles into the water pump housing which reduces efficiency and you will get an overheated engine.

KJ
 
Okay thank you.

There is a pin that holds my outboard at a certain tilt and there are about 5 different settings.

Do you think moving it to a lower angle pin may help with the cavitation?
 
Yes. With a fixed tilt like that it’s usually the 2nd hole position from the transom that’s a good compromise between holeshot performance and bow angle when on plane. Sometimes the 3rd hole is okay but it gives a lot of bow lift out of the hole.
 
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