Engine Over-Rev but Loss of Thrust

[sirrufus]

I have a mid-90's Evinrude 9.9 that is acting like it has a spun hub. After giving it a little throttle the engine rpms will rise but it will lose most thrust. It will move very slowly at just above idle speed. I researched the forums and marked and checked the prop which checked out fine. Next I thought about ventillation, but the boat/motor was working OK before this problem so I don't think it is ventillation. Everything else seems to be fine. What do I check next?? Any ideas?? Thanks!!:

 

[bktheking]

Re: Engine Over-Rev but Loss of Thrust

Did you change boats???? If I were you I'd drop the gearcase and look at the driveshaft spline that goes into the powerhead, could be stripped.

 

[sirrufus]

Re: Engine Over-Rev but Loss of Thrust

No - same boat. I got it used, but it was working fine that's why I didn't think it was a ventillation problem. I will check the drive shaft splines. Thanks!

 

[jtexas]

Re: Engine Over-Rev but Loss of Thrust

9.9 with thru-hub exhaust? if not it has a shear pin instead. take the prop off & see what condition it's in.

other possibility is cavitation caused when something blocks the flow of water into the blades...L/U or prop damage, a bit of vegetation, etc.

 

[bktheking]

Re: Engine Over-Rev but Loss of Thrust

Ventilation problems don't happen "all of a sudden" unless something was changed.

 

[matt1982]

Re: Engine Over-Rev but Loss of Thrust

start at the prop and work towards the motor... can you put it in gear with the motor off and spin the prop? check prop pin, drive shaft, lower unit gears etc.