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I was driving my 13 on the river yesterday when my 25hp Nissan 4-stroke started acting up. I would be on plane when the OB would loose power and slow way down and rev way up. This happened several times throughout the day. I would describe it like engaging the clutch while still pushing on the gas when driving a stick shift.
I did find a wire (brown) that was exposed but when running it in the yard and grounding it out I could not get the same results.
What could be the problem?

Thanks!

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You don't mention how old the motor is. Could be a carb problem.

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If I understand correctly the boat slows down while the engine revs up. That would indicate slippage somewhere between the power head and the prop. Could be the prop hub torn. Had any run ins with sticks or the bottom? Could also be lower end gears.
I'm not familiar with you're engine so these are only guesses.

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I think Tig has a good point.

There is a center hub or shear pin on the propeller and it is designed to limit gearcase damage (by breaking) if you hit something with the propeller.

If you can, with the engine turned OFF, put it in forward gear and try turning the prop by hand. If it turns, it may be the hub or shear pin has broken.

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Rigging maybe?

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No, no run ins with rocks or the bottom...only a thick patch of weeds the time out before this started to happen.

The OB is a 2006.

When I get home I will go turn the prop by hand...this sounds like it could be a center hub or shear pin problem. It def feels like a gear slipping. There is no grinding noises so hopefully its not to bad.....but we all know how that goes, the ones that sound bad and scare the sh"t out of us are no problem its the quiet "easy" fixes that get us.

THANKS GUYS!

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YEP, with the engine in forward gear I can spin the prop by hand. In reverse it is rock solid. When I spin the prop it makes a click every 1/2 turn or so. Now what?

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Sounds like the prop and shear pin are OK if it is solid in one direction. Engine is quite new so I don't think the gears are worn out. It could be that the shift linkage is loose/damaged and is not engaging the forward gear completely. Or more likely something failed inside the lower end where the gears are. Clicking every half turn is odd. I would expect a distinct skipping if the gears were not meshing.
Check the linkage top end and also look for an inspection plate, which is where the linkage is coupled to the lower end. If that's OK (not loose or sloppy) remove the lower end oil drain plug and see what comes out. Water? Metal filings?
It may be time to get a repair manual. The lower end isn't very complicated but the manual will show you how it all comes apart and goes back together.

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The "clicking" he mentioned makes me think it's a cable, or other adjustment, like "Tig" said, drain the lower unit and check the oil, if it's "Milky" their's water in there.

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modenacart mentioned rigging and Matt added cable. Very good thoughts. If your helm controls were out of adjustment, damaged or perhaps binding they could cause the engine to not fully engage the gear. Disconnect the controls from the engine, shift the motor by hand at the engine and retry the prop test.

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The boat would slow down but the engine would rev up? That doesn't sound like the engine was loosing power. To me it sounds more like weeds or a plastic bag around the prop.

Next time pop it in neutral and once you slow down try HARD reverse. Make an arc around the area your boat just was so you don't run over whatever may have come off and try popping back up on plane again.

Edit: Upon reading the free spinning when in forward but not reverse, I second checking the cables to make sure you are fully engaging the gears.

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I second what Tig said, disconnect linkage and try to shift engine by hand, then turn prop. If this is good, then follow the procedures to adjust the linkage.

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Third that, and when you're moving the shift manually, pay attention to the feel and distance from neutral to reverse and then neutral to forward.

Was your lower unit taken off recently for any reason?

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Just talked to my mechanic, he said it sounds like the shift linkage is loose or the prop hub has broken loose causing it to spin.
So tonight I will go look at the shift linkage before taking it to be fixed.

Thanks again guys!

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When you put the prop on, there is a torque spec for it too.