Re: Merc 1976 500 stuck in forward previous owner replaced water pump did he incorrec
First question, is: After his repair, did it ever work properly?
If no, then he probably didn't get the shifter spline lined up properly...he didn't have the shifter in the lower unit in the same gear as he had the shifter control and in doing so, when he engaged the splined shafts (lower unit to mid section) one wouldn't let the other go all the way too one end or the other and you lost the ability to shift to either F or R.
Or the spline shafts weren't even connected so even though the mid section female spline moved as desired with your comment in your post, the male spline in the lower unit remained where it was when he reassembled them.
So, yes, he could have reassembled it incorrectly.
If yes, you may have a broken/frozen shifter shuttle gear in the lower unit.
Mark