higher speed engine revs forward motion drops off

[MistyTazzvon]

1989 Sunbird Corsica with OMC 3.0 Cobra inline 4 and OMC Cobra out-drive. Really hard to explain but at slow and medium speed everything is fine but when i go to speed up to plane it gets to a point (i think around 3-4k rpm not positive) it is almost like the drive disengages and the engine starts revving up. Engine seems fine and as soon as it does it i back off the throttle and slow for a bit then try again but same results every time. There are some decent nicks in the prop (bought it that way) and this is the first time i have every taken this boat out. Any suggestions would be great and detailed explanations of how to test for any problems would be nice too. I had a couple of thoughts (forward thrust or bushing) but not sure how to test for anything.

Thank you for any suggestions.

 

[Tnstratofam]

Not sure but it almost sounds like a spun hub in your prop. If the hub is damaged it may work okay at low to medium speed and then lose the ability to provide forward motion as the hub out spins the prop so to speak. Especially if the prop has sustained a good hit it could have damaged the hub.

 

[GA_Boater]

It sounds like you have a spun prop. Use a marker and draw a line across the prop and hub, then take to boat out one more time. After duplicating the problem, look at the line. If it isn't still straight, it's the hub. It can be fixed by a prop shop or by replacement.

 

[MistyTazzvon]

Hate to ask but i just don't know, what is the hub since i don't see anything on the diagram in the book marked hub. I think it's the center part of the prop but could you please let me know?

 

[Baylinerchuck]

Yes center part of the prop, with the rubber insert.

 

[MistyTazzvon]

thank you for the quick responses. i will try to take it again tomorow and mark it prior.