outboard over reving.

My 25 hp. Johnson outboard sometimes starts to over rev when on full throttle. I back it off and it’s ok and at full bore it will over rev again.I know it sounds just like a spun prop would act, as I have had this on another motor. I replaced the old prop with a new, or maybe it is a rebuilt, I’m not sure but it acts the same. My ??? is, is there any other thing that can cause an outboard to slip between the prop and the powerhead?? I look closely at the prop hub and it doesn’t have any taers or obvious reason to slip. I was careful to break this prop in according to the dealers instructions. I never hit nothing, honest.

My Johnson 30 did that some too. Turned out I was missing a plastic peice that fits around the outside hub of the prop. This peice directs the exhaust in some way. I found it happened more in any kind of waves. 25 bucks at the prop shop and it was good to go. You are experiencing cavitation I presume.

Exactly, excellent response. Thank you, I’ll look into that concept.

If you’ve had the prop off did you put the prop washer back on?

sounds like you got the wrong washer or missing the plastic piece.