I think you have a sheared pin behind the prop. It may even look fine at first glance...Until you go to remove it,and find it is in three pieces.

Couldn't hurt to check tho, as this is the part DESIGNED to fail,to protect the rest of the gear train.

But if it doesn't come out in three pieces, you may have to pull the gearcase and check the shift dogs as Yepblaze sugested. Andd you'll need the book for that.