| Author |
Message |
    
Paul Speidell
Member
Username: snospdr
Post Number: 11
Registered: 08-2008
| | Posted on Saturday, November 07, 2009 - 02:04 pm: | 
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I have a 1979 70 hp Johnson on a pontoon boat. After replacing a faulty rectifier and old plugs, my engine seemed to run great - except when I brought it up to over about 4000 RPMs. At that point the engine RPMs would increase, but the boat would slow down, as if the prop was cavitating. It would continue to run this way until I pulled back on the throttle. Then I'd begin to get propulsion again. When increasing the throttle again, but slowly, the same thing would happen. So, is this a hub slipping problem? I removed the prop, could not determine that the inside portion that accepts the splined prop shaft was loose in any way. I did wonder if the disk that sits just behind the prop (has a rubber coated side) could be causing the problem.
Thanks in advance for any suggestions! |
VicS
Advanced Member
Username: vics
Post Number: 743
Registered: 05-2008
| | Posted on Saturday, November 07, 2009 - 03:56 pm: |
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Your symptoms are exactly those of a spun prop
Mark a line across the end of the prop shaft and the prop. Give it a run until you observe the symptoms. Check the marks. If the marks on the prop no longer line up with the line on the shaft end its a spun prop! |
Morten Ringvold
Senior Member
Username: haffiman37
Post Number: 4346
Registered: 04-2006
| | Posted on Saturday, November 07, 2009 - 06:41 pm: |
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' I did wonder if the disk that sits just behind the prop (has a rubber coated side) could be causing the problem. '
It does NOT have a 'rubber coated' side. more likely melted rubber from a spun hub! |
Paul Speidell
Member
Username: snospdr
Post Number: 12
Registered: 08-2008
| | Posted on Sunday, November 08, 2009 - 12:36 am: |
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Okay. Thanks for the very helpful info.! |
Oscar Ansley
Member
Username: bass_tracker
Post Number: 18
Registered: 11-2008
| | Posted on Sunday, November 08, 2009 - 04:54 pm: |
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Could be sucking air around the prop if it's tilted to far up. |
Paul Speidell
Member
Username: snospdr
Post Number: 13
Registered: 08-2008
| | Posted on Sunday, November 08, 2009 - 08:19 pm: |
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Thanks. It's straight down, so I think it's the hub. |
