2000 johnson 50hp cavitation but not prop

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bcasurvey

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Dec 14, 2011
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I just bought a 2000 pontoon boat with a Johnson 50hp 2 stroke. (without test driving it).
I had a cavitation problem so I bought a new prop thinking the hub was spinning, but it wasn't the prop causing problem.
when changing the gear oil I noticed there is water in the oil.
1). can water in the gear oil cause the prop to spin (cavitate)? it only does it at 3/4 throttle and higher.
thanks for any help...
 
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Will Bark

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Re: 2000 johnson 50hp cavitation but not prop

Is the motor set deep enough in the water? Sounds like it needs to be lowered.
 
pn

pn

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Re: 2000 johnson 50hp cavitation but not prop

can't answer your question but water in the lu is no good.
 
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bcasurvey

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Re: 2000 johnson 50hp cavitation but not prop

the motor height is original factory set and is as low as it can go. without cutting on the transom ect..
I bought it not running but I wouldn't think the factory would send it out that way?
ive never owned an outboard so I don't know if theres some kind of clutches or something that would cause it to slip if the oil got thinned out with water. I need to find out why its spinning then my next question is what the best way to find out how water is getting in the oil.
can it be condensation if it was sitting in the water for a year? it wasn't full of water but it was milky.
 
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bcasurvey

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Re: 2000 johnson 50hp cavitation but not prop

the motor height is original factory set and is as low as it can go. without cutting on the transom ect..
I bought it not running but I wouldn't think the factory would send it out that way?
ive never owned an outboard so I don't know if theres some kind of clutches or something that would cause it to slip if the oil got thinned out with water. I need to find out why its spinning then my next question is what the best way to find out how water is getting in the oil.
can it be condensation if it was sitting in the water for a year? it wasn't full of water but it was milky.
 
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boobie

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Re: 2000 johnson 50hp cavitation but not prop

Do you have a lot of people on the boat when you're running it or is it just you ?? To find out about the water in the g/c have it pressure and vacummed checked.
 
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bcasurvey

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Re: 2000 johnson 50hp cavitation but not prop

no just me in the boat.
I guess ill pull it and take it to someone but I was hoping I could do it myself.
 
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bcasurvey

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Re: 2000 johnson 50hp cavitation but not prop

does anyone know if water in the gear oil will cause the prop to break free and spin?
 
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multimech

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Re: 2000 johnson 50hp cavitation but not prop

No, because it has a mechanical clutch. Eventually with water ( a poor lubricant) in the case you could get enough wear to have it jump out of gear, but not spin free. You will need to have it resealed in any case.

Factory specs don't count worth beans in boats. :) Check the transom height and make sure the model engine you have fits the transom. For example a 25" in a 25" transom. With pontoon boats there is no way to check the height at the bottom of the boat and they have different characteristics to planing boats. I am betting there is a problem with either water flow under the hull from a design perspective or maybe something, like a sender, in direct line with engine.

How did you check the prop to see if the hub was spun or not?
 
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bcasurvey

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Re: 2000 johnson 50hp cavitation but not prop

when I say spin free I really mean it jumps out of gear. its more like after planning out when I go from 3/4 throttle to full throttle it jumps out of gear when I throttle back down it catches again and will stay hooked up until I floor it again.
remember the boat is 13 years old, no sender in the way.
I replaced the prop with a brand new one and it didn't help.
I was wandering if my motor had dog gears and if they could be wore out?
also how can I tell the length of my lower unit?
 
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bcasurvey

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Re: 2000 johnson 50hp cavitation but not prop

the model # is j50plssm

I just drove it again and it breaks free around 4500 rpm's and just free spins until I back off the throttle then it catches again
 
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bcasurvey

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Re: 2000 johnson 50hp cavitation but not prop

If the shift cable was out of adjustment could it cause the problem?
 
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bcasurvey

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Re: 2000 johnson 50hp cavitation but not prop

can you tell the lower unit shaft length by the model number j50plssm
also is there a way to measure the shaft length with a tape measure if so where do I measure from?
 
Tim Frank

Tim Frank

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Re: 2000 johnson 50hp cavitation but not prop

That model number is a 20" shaft.
 
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Scurvy Knave

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Re: 2000 johnson 50hp cavitation but not prop

just out of curiosity, when it "catches again" as you throttle back , does it jerk violently? Does it cause the motor to shudder? I'm guessing a clutch slipping in and out around 4500 RPM would produce quite the shock, and possible severely damage the prop shaft splines, clutch dogs and forward gear.
 
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bcasurvey

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Re: 2000 johnson 50hp cavitation but not prop

no it doesn't jerk or anything, I'm thinking its cavitating air somehow, after looking again today. the prop doesn't slow down on rpm's it increases. im trying to upload pics now.
 
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bcasurvey

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Re: 2000 johnson 50hp cavitation but not prop

no it doesn't jerk or anything, I'm thinking its cavitating air somehow, after looking again today. the prop doesn't slow down on rpm's it increases. im trying to upload pics now.
 
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bcasurvey

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Re: 2000 johnson 50hp cavitation but not prop

lower1a.jpglower2a.jpglower3a.jpg

I'm wondering if the "v" plate at the bottom of the transom is causing the problem?
 
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