1973 Johnson 25 acting like prop hub spun...

[Howard Sterndrive]

but it hasn't spun. I tried several props. It does it the worst with the 12P. almost held on plane with a 10P, but then slipped.

If the dogs in the lower were gone, I would expect clunking. But this feels exactly like a spun prop hub.
Holds torque until just about to plane then revs up.

Could there be a spline somewhere slipping?
not familiar with how these lowers connect to the powerhead... but I suspect I'll be finding out here soon.

besides the prop I don't see where it could slip... no signs of anything going on with the shear pin...looks fine

not a ventilation issue or anything like that- I have my identical 1975 25HP on the boat now. I put the same props on it - no problem on the '75

 

[restornator]

Re: 1973 Johnson 25 acting like prop hub spun...

you said you wouldn't notice a slipping clutch dog. it can act just like a spun hub, and without any resistance from the propeller, it will rev up. best to turn the clutch dog around (if reverse is okay) adn try again. If it acts the same, you are looking at forward gear (or pinion gear, drive shaft, or crankshaft)

 

[Howard Sterndrive]

Re: 1973 Johnson 25 acting like prop hub spun...

you said you wouldn't notice a slipping clutch dog.

I said I would expect a clunk

wouldn't that clunk abruptly?

this is butter smooth

 

[lindy46]

Re: 1973 Johnson 25 acting like prop hub spun...

Did you actually confirm that the hubs are not spinning by drawing a line across the prop/hub? Maybe you have two spun props.

 

[Howard Sterndrive]

Re: 1973 Johnson 25 acting like prop hub spun...

I noticed afterward, that one of the props was indeed scribed. the marks were aligned
plus they don't slip on my other 25
can't see why a prop would slip on one identical motor and not the other

 

[Andy in NY]

Re: 1973 Johnson 25 acting like prop hub spun...

is your motor too high or trimmed out too much? could be ventilating.

 

[jtexas]

Re: 1973 Johnson 25 acting like prop hub spun...

is your motor too high or trimmed out too much? could be ventilating.

yup. or cavitating. any kind of damage or irregularity on the lower unit, if it blocks the flow of water into the blades, will cause cavitation.

or if a piece of the antiventilation plate is broke off.

 

[Howard Sterndrive]

Re: 1973 Johnson 25 acting like prop hub spun...

I'll pull the lower off this weekend

must be the vertical shaft into the powerhead

 

[Howard Sterndrive]

Re: 1973 Johnson 25 acting like prop hub spun...

is your motor too high or trimmed out too much? could be ventilating.

can't see why a prop would slip on one identical motor and not the other
same pin hole