40 Yamaha Vibrates and Stops Moving Forward

[iammarcuse]

1984 Yamaha 40hp 2 stroke. It is a friends boat and I am going out with him on the water this week. It starts and runs fine, but all of sudden the motor starts to vibrate louder and louder and the boat slows down and comes to a stop. I stopped by his house and the prop looked like it had a spun hub and so I suggested he change it, which he did. He is saying it is still doing the same thing. Wondering what you guys think I should be looking for when we go out. I have worked on a lot of boats but this is a new one for me? He says it is not popping our of gear and it has the new prop. Something up with gear case?

 

[WhyNaut]

Clutch dogs in lower unit could be bad.

 

[robert graham]

Drain lower unit oil to check for metal pieces in it....if water got in there it could have ruined the lower unit....especially if it was salt water...