Re: 76 Evinrude not shifting into forward...
Not sure if you've messed with your cables or not, but I had this same problem recently when I moved my 115HP johnson from one boat to another.
Turns out I had the trunnion nut adjustment screwed up and when I corrected it my problem was gone.
Like metriccrescentwrench said, it would be a good place to start if you haven't done it already.
Here's the instructions I followed per Joe Reeves:
Re: ??? about 1974 115hp HYDRO-MECH shift
(Centering Shift Cable)
(J. Reeves)
When all is as it should be, the proper method to adjust the shift cable is to disconnect the cable from the engine. Move the shift linkage on the engine to find the center of the play in neutral, and when found, leave it centered.
Now, grab the end of the shift cable sleeve, push and pull it to find the center of the play there, and center that play.
Adjust the trunion on the threaded portion of the shift cable so that the centered play of the cable lines up with the centered play of the engine's shift linkage. Install and lock the shift cable with the retaining clamp in that position. That's it.