Motor question?

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Motor question?

Postby rookiey1 » Sat Nov 16, 2013 3:22 pm

I have a 25 h.p. Johnson on a 16 ft boat and every once in awhile when traveling wide open it will rev real high and stop going as if the prop fell off if I let off the throttle for a second it will work again almost as if it is sucking air. Anybody ever have this happen or any ideas as to what the problem is ?
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Re: Motor question?

Postby z51 » Sat Nov 16, 2013 9:00 pm

Try moving the trim pin position and see if that helps.
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Re: Motor question?

Postby yareelohim » Sun Nov 17, 2013 9:58 am

When you get a chance, a drain a little oil from the lower unit and see if any water comes out.

It's possible you have a slight water leak in a seal and are getting small amounts of water over time to cause slipping.

If that's not the case then your motor may be mounted too high or trimmed too high.

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Re: Motor question?

Postby Smith » Sun Nov 17, 2013 1:34 pm

Your propeller probably has a pressed-in rubber hub that is supposed to slip under extreme load, similar to the function of a shear pin. These hubs can lose their compressive force and start slipping even under normal loads. If that is the case here, an outboard shop or a propeller shop can press a new hub in for you. Last time I had it done, it wasn't expensive.
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Re: Motor question?

Postby Boatman » Sun Nov 17, 2013 5:50 pm

Mark the prop and inner hub with a punch. Run it till it slips and see if the marks still line up.
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Re: Motor question?

Postby T Man » Sun Nov 17, 2013 8:04 pm

Boatman wrote:Mark the prop and inner hub with a punch. Run it till it slips and see if the marks still line up.


This. Sounds like a spun hub to me too.

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Re: Motor question?

Postby rookiey1 » Sun Nov 17, 2013 8:44 pm

Thanks everyone, think I'll try marking hub and prop first then working my through the suggestion's until I figure it out. I'll post result....
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Re: Motor question?

Postby sureshot 612 » Mon Nov 18, 2013 1:19 pm

let me guess you have a 16 ft boat and the prop pops out of the water . i have a 30hp even. it does the same thing lower the motor down with the trim pin ,i need to set mine on the 2nd one up. on 3 it won't run right .
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Re: Motor question?

Postby tmclaimerFL » Mon Nov 18, 2013 2:57 pm

T Man wrote:
Boatman wrote:Mark the prop and inner hub with a punch. Run it till it slips and see if the marks still line up.


This. Sounds like a spun hub to me too.

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This or possibly cavitation
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Re: Motor question?

Postby lostknife4 » Mon Nov 18, 2013 5:39 pm

Smith wrote:Your propeller probably has a pressed-in rubber hub that is supposed to slip under extreme load, similar to the function of a shear pin. These hubs can lose their compressive force and start slipping even under normal loads. If that is the case here, an outboard shop or a propeller shop can press a new hub in for you. Last time I had it done, it wasn't expensive.

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Re: Motor question?

Postby threedogs » Tue Nov 19, 2013 12:37 pm

Hubs are not supposed to slip under extreme load , the rubber hub is a fail safe for hitting obsacles to keep you from breaking a prop shaft. Once that has been done it will never hold it's bite correctly again and needs to be rehubbed.
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Re: Motor question?

Postby brock2013 » Wed Nov 20, 2013 6:30 pm

I have the same motor and mine was doing the same thing when my prop was worn out. I put a new prop on it and it hasn't done it since.
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Re: Motor question?

Postby Smith » Thu Nov 21, 2013 5:43 am

threedogs wrote:Hubs are not supposed to slip under extreme load , the rubber hub is a fail safe for hitting obsacles to keep you from breaking a prop shaft. Once that has been done it will never hold it's bite correctly again and needs to be rehubbed.


Hitting an obstacle is exactly the sort of extreme load I was talking about. That's why I said "similar to the function of a shear pin". I don't know if slipping once ruins it or not.
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Re: Motor question?

Postby lostknife4 » Fri Nov 22, 2013 2:07 pm

A lot depends on how long it was run while it was slipping before someone backed off the throttle to let it grip again. It could have worn it enough that it would slip easily on full acceleration or even at high speed if it was worn enough.
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Re: Motor question?

Postby bknilvr00 » Fri Nov 22, 2013 3:16 pm

I've got a 30 horse merc that's barely two years old that was doing the same thing. It has an aluminum prop on it, and the edges were pretty gnarly. No bent blades or anything like that. Just some jagged edges. I took a file to them to smooth them out and I haven't had the problem since. Rough edges cause a lot of cavitation. After this season I want to switch to a stainless prop, but an aluminum one is more forgiving without blowing though shear pins.

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Re: Motor question?

Postby yareelohim » Sun Nov 24, 2013 8:36 am

Another note to consider...

If you run the motor through weed beds and pickup enough weeds on the prop, you will experience what your describing.

This happens to me every time I launch in the river when the water is low. I simply through the motor in reverse, gun it and let the motor pop up out of the water, that usually clears all the weeds off.
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