Spun prop How to tell

"I have a 1979 70 hp Johnson o

"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!"

Your symptoms are exactly thos

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!

' I did wonder if the disk

' 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!

Okay. Thanks for the very hel

Okay. Thanks for the very helpful info.!

Could be sucking air around th

Could be sucking air around the prop if it's tilted to far up.

"Thanks. It's straight dow

"Thanks. It's straight down, so I think it's the hub."