shamanic wrote:
It might just be the rubber spinning inside of the prop.I had that happen on a old 9.5 and I thought it was a problem with the gears but a new prop fixed it up.
1976 still uses shear pins. No rubber hub. My guess is the linkage rod has loosened and will not completely engage either forward or reverse. Half way down the lower unit there is an elongated oval inspection plate. 2"x1/2" held by two screws. Remove the cover and you will see a union. The linkage rod is in two pieces held together by this union. The gear case cannot be removed without separating the linkage rod at this point. If it has stripped or loosened the linkage rod will not travel far enough to engage in either direction.
It's been a while but it seems to me both the shifter and the gear case have to be in neutral with the union loose. Once you establish that they are both in neutral, the union is tightened.
I had a 1974 Johnson 25 and experienced the same problem after replacing the water pump. When I put everything back together, the linkage was not adjusted properly and it would jump out of gear.
Hope this helps.