69 Johnson 25

[MikeGo]

Gets up on plane for a time then revs high and falls off plane.

 

[flyingscott]

Try a different prop

 

[tphoyt]

I think we are going to need more info.
Is this a new problem?
Is the boat new to you?
What kind of boat?
Things like that would be helpful.
What does the hull look like does it need cleaned? Could be causing cavitation.

 

[MikeGo]

I think we are going to need more info.
Is this a new problem?
Is the boat new to you?
What kind of boat?
Things like that would be helpful.
What does the hull look like does it need cleaned? Could be causing cavitation.

It is new to me. It will get up on plane and stay there for say 20 minutes or so and then the engine will rev real high and it will drop off plane. It seems like the prop is still turning however. After that sometimes you can start real slow and slowly build speed and it will get on plane and stay there for a short time. It is a tiller drive 14.5 ft flat bottom fishing boat that is not extremely heavy.

 

[MikeGo]

It is new to me. It will get up on plane and stay there for say 20 minutes or so and then the engine will rev real high and it will drop off plane. It seems like the prop is still turning however. After that sometimes you can start real slow and slowly build speed and it will get on plane and stay there for a short time. It is a tiller drive 14.5 ft flat bottom fishing boat that is not extremely heavy.

It seems to work well at first, then the more you use it the more it won't want to get on plane or stay on plane.

 

[F_R]

It is either a slipping prop hub or it is cavitating. It is easy to tell if it is the prop hub. Make a mark on the prop nut (or easier yet use the cotter pin as a pointer). Make a second mark in the propeller, aligned with the first mark (or cotter pin). Run the motor till suspected slippage occurs. Stop motor and look at your marks. Are they still aligned? If no, it has slipped. If they are still aligned, repeat test. There is an off chance that it stopped slipping in the same position.

If it isn't the prop hub, check boat for cavitation causes.

 

[MikeGo]

It is either a slipping prop hub or it is cavitating. It is easy to tell if it is the prop hub. Make a mark on the prop nut (or easier yet use the cotter pin as a pointer). Make a second mark in the propeller, aligned with the first mark (or cotter pin). Run the motor till suspected slippage occurs. Stop motor and look at your marks. Are they still aligned? If no, it has slipped. If they are still aligned, repeat test. There is an off chance that it stopped slipping in the same position.

If it isn't the prop hub, check boat for cavitation causes.

Thank you I will try that. I tried to turn the prop hub with a wrench while it was off the motor. I torqued on it pretty hard and could not get it to move.

 

[MikeGo]

Thank you I will try that. I tried to turn the prop hub with a wrench while it was off the motor. I torqued on it pretty hard and could not get it to move.

 

[MikeGo]

Thank you I will try that. I tried to turn the prop hub with a wrench while it was off the motor. I torqued on it pretty hard and could not get it to move.

Going to order a new prop I don't know if I should get a nine or a 10.

 

[tphoyt]

Getting the prop to slip by hand is very difficult unless it’s completely toast.
FR’s slip test is an important test to properly diagnose for slippage.
Check around with local shops to see if any will let you try a couple different props.
Then buy from them and support you local businesses if you can.

 

[oldboat1]

...and the business that runs this website (iBoats).

Maybe the motor is too high on the transom. Start with the anti ventilation plate even with the bottom, and adjust from there if needed.

 

[MikeGo]

...and the business that runs this website (iBoats).

Maybe the motor is too high on the transom. Start with the anti ventilation plate even with the bottom, and adjust from there if needed.

Okay so I bought a 9 and 1/4 by 11 pitch prop from Michigan match. It works better than I could have imagined. The boat and motor now perform beautifully. Now the next problem I have run into is I took it out onto Manistee lake in Western Michigan this weekend. Used it for about 5 hours parked it in the driveway. After a while I began to notice oil like substance leaking out the exhaust. I found out that it was due to my gas tank being overpressurized. When I took the cap off the tank I am guessing there had to be at least 10 PSI in there. How could that tank get pressurized so much it was not in the sun.

 

[oldboat1]

Nothing more than unburned fuel draining out. Not a problem. Unhook the tank at the motor if you that that is causing an issue. Great fishing area.