Mercury 25 hp Reverse Not Engaging

[BOATMAN22424]

I just got a late 80's Mercury 25 HP outboard. It is tiller handle steer. It engaes into nuetral, and forward smoothly, but when I engage into reverse the prop turns, and sounds like it is grinding some. The prop does turn, but not as fast as it does in forward, and it will not even move my boat backwards at all, no matter how much i accell into the reverse gear. Can anyone help me out with what I could do, and or look at? Any comments would be very appreciated.