Prior to 1993 Yamaha used regular steel, not stainless steel, for the shift
rods that ran from linkage under the carburetors down to the gearcase.
Mercury had the same system years and years ago, and Yamaha "copied" the
Merc gearcase design. These rusted away over time and as the diameter got
thinner as the rust flaked off, it turned into a "torsion bar" and would not
shift out of gear while under load. They would twist but not turn the
gearcase shift shaft.
If you look at the rod at the opening near the lower mounts, you may see
where the diameter has corroded away from about 1/4 " to around 1/8". The
bad part is that you have to remove the powerhead to replace the shaft. On
an engine that old in saltwater, it may be almost impossible to remove the
long mounting bolts holding the powerhead onto the midsection.
This is assuming that you reinstalled the shift rod splines correctly and
there is no internal problems with the gearcase.
Bill Grannis
service manager
Maybe you just need to adjust the linkage. My Mariner 15 (Yamaha) has an
adjustment nut in the middle of the leg. Also where you split the linkage
when dropping the leg.
Bill