I have a 1973 115hp outboard that only goes in reverse and neutral when i shift it into forward in pulls in reverse.

Thank you for the question,

This unit is a Hydraulically assisted shift lower Gearcase....

Though the shift is mechanical in nature as far as the control box, shift cables and the linkages down to the lowerunit is concerned. The actual movement of the clutch is done hydraulically within the lowerunit. Look closely at the diagram to see the components.

To begin troubleshooting this system we need to confirm that the cable is moving equal distances from neutral under the cowling. Have a friend moving the control handle on the dash from neutral to forward and back then from neutral to reverse and back. While you look at the shift lever under the cowl and measure that it is moving the same distance in each event. Once this is confirmed it off with the lowerunit for inspection.

If the problem is found to be in the lowerunit, and the controls are moving the shift lever the proper distance in each direction. Exploratory surgery is the only way to find which o-ring or component has failed to cause this issue.

Removal of the lowerunit is straight forward....As the replacement of the rubber impeller for the water pump is just above the lowerunit....

This lowerunit disassembly will require several special, but not OMC made tools....A deep pair of large snap ring pliers an extensions with sockets, A harmonic balancer puller and two long threaded bolts will be needed....(OMC does make the "Special" tools also, but these can be gotten around)

The lowerunit is complex, and not straightforward as to it's operation, but with a good manual and meticulous mechanical habits, you should be able to get through the repair.