1972 Johnson 25 hp "slipping" problem

buknut · May 29, 2007

I'm running a 25 hp 1972 Johnson on a 14' semi v haul. The engine seems a little heavy for the boat, but it runs great. However, this past weekend, I could only run at about 1/3 - 1/2 throttle. If I opened it up, the engine would sound like it came out of gear or peel out, but of course it didn't. I've read some other threads about a prop hub slipping problem, which sounds like it could be it. I'm not sure the age of the prop, but it looks to be the original. There was no out of the ordinary jerking or grinding that I could notice. I drained the oil and it looks fine. Does this engine have a prop that could slip? I took it off and I can't see any rubber hub. If it is, any help on a source for a new one would be appreciated

larky · May 30, 2007

Re: 1972 Johnson 25 hp "slipping" problem

yes it does sound like the prop hub. you cant see it but its in there. you will have to send it away to have it repaired or if you have a local prop shop, drop it off. not sure of who or where, but should be cheaper than buying a new prop

tashasdaddy · May 30, 2007

Re: 1972 Johnson 25 hp "slipping" problem

you can see in this picture how the prop is made, your is the same principle, but not the thru hub exhaust. there is the shiney outer ring, the rubber hub, and the inner shiney ring .

buknut · May 30, 2007

Re: 1972 Johnson 25 hp "slipping" problem

Great thanks. I was hoping that was the problem (and not the clutch dog). I'll call around for a prop shop in the area.

tashasdaddy · May 30, 2007

Re: 1972 Johnson 25 hp "slipping" problem

or clik johnson props at the top of this page, have a new one and then have the old one repaired for a spare.

Bindernut · May 30, 2007

Re: 1972 Johnson 25 hp "slipping" problem

A 72 25hp will still be a pin-drive...no rubber hub insert. I don't remember ever hearing about one of them slipping unless the prop isn't seated on the pin all the way (or the pin is broke). I guess it might be possible for the aluminum prop body to be spinning on the brass drive insert but I've never seen that happen.

If the drive pin is broken, save yourself some hassle and get "real" (as in solid) drive pins instead of just using roll pins. Learned that the hard way on a friends pontoon ('73 25hp Johnny)...had carb trouble with it a couple years ago. Every time it would sneeze, it'd shear the pin. He was using roll pins. We borrowed a solid pin from a neigbor when we were debugging the carb. Even after several more sneezes, that pin is still in one piece. And he's got solid spares now too just in case this one ever shears.

Another thing that might cause the slipping at higher throttle settings...
The prop might be cavitating at higher speeds. If it sucks air, it can feel like the prop "let go".

xxCARPxx · Sep 19, 2008

Re: 1972 Johnson 25 hp "slipping" problem

I would like to know if the new prop fixed the problem....... I have a 85, 16' semi V with a Johnson 25 the does the same thing..(acts like the prop lets go at higher speed)...and I have removed the prop..but didnt see any sheared splines...or a inner hub......

jbjennings · Sep 20, 2008

Re: 1972 Johnson 25 hp "slipping" problem

If you're wondering if your hub is slipping, just mark it with a permanent marker across the prop shaft and take it for a short run until it messes up. IF your marks don't line up afterward, then your hub is slipping. If the marks are still lined up, you've got a bad clutch dog/forward gear ears. Tashasdaddy has a nice pic I'll bet he'll post to show you how to do it if you don't understand fully. You'll probably have to take your prop cone off to mark it correctly but that's no problemo.
Good luck and I hope it's the hub!

JBJ

xxCARPxx · Sep 20, 2008

Re: 1972 Johnson 25 hp "slipping" problem

I have removed the prop....I have been explained that the "hub" in rubber...The only thing I see that might be a hub looks like it's brass....I have marked it on both the inside and out side of the prop and will try it out in a few......Also on the outside of the prop the number "012 016" is stamped on it..what is that????I'll TRY to upload some pics of the prop,in hope that some one might be albe to describe the hub for me....thanks for any..and ALL help..

freddyray21 · Sep 20, 2008

Re: 1972 Johnson 25 hp "slipping" problem

Bindernut said:
A 72 25hp will still be a pin-drive...no rubber hub insert. I don't remember ever hearing about one of them slipping unless the prop isn't seated on the pin all the way (or the pin is broke). I guess it might be possible for the aluminum prop body to be spinning on the brass drive insert but I've never seen that happen.

QUOTE]

those still have a hub even though they are pin drive.

xxCARPxx · Sep 20, 2008

Re: 1972 Johnson 25 hp "slipping" problem

I marked it..put it in gear..and could turn it by hand,with out even having to go to the water...Thank you all so much for your help....

samo_ott · Sep 20, 2008

Re: 1972 Johnson 25 hp "slipping" problem

Wow, that really was spun. I'm surprised you got up to half speed with it!

freddyray21 · Sep 20, 2008

Re: 1972 Johnson 25 hp "slipping" problem

you can turn the prop in gear. It also turns the motor. However you are not going to be generating the torque the motor will. It's hard to tell if a prop hub is spun by turning it by hand. A new prop will tell you a lot.

Johnny Ringo ! · Jun 30, 2023

Bindernut said:
Re: 1972 Johnson 25 hp "slipping" problem

A 72 25hp will still be a pin-drive...no rubber hub insert. I don't remember ever hearing about one of them slipping unless the prop isn't seated on the pin all the way (or the pin is broke). I guess it might be possible for the aluminum prop body to be spinning on the brass drive insert but I've never seen that happen.

If the drive pin is broken, save yourself some hassle and get "real" (as in solid) drive pins instead of just using roll pins. Learned that the hard way on a friends pontoon ('73 25hp Johnny)...had carb trouble with it a couple years ago. Every time it would sneeze, it'd shear the pin. He was using roll pins. We borrowed a solid pin from a neigbor when we were debugging the carb. Even after several more sneezes, that pin is still in one piece. And he's got solid spares now too just in case this one ever shears.

Another thing that might cause the slipping at higher throttle settings...
The prop might be cavitating at higher speeds. If it sucks air, it can feel like the prop "let go".

That was what I was thinking, it may have been cavitating.

racerone · Jun 30, 2023

Those props have a rubber hub !!

1972 Johnson 25 hp "slipping" problem

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