It could be expensive ( worn gears ) or more likely cheap( detent spring ).
There is a detent spring which pushes a ball bearing into a notch on the
linkage to hold the shifter position when in gear. Over the years these
springs get weak and don't push the ball into the notch or the ball is all
gooped up and doesn't move easily. These are accessible from the top of the
trany. There is a 9/16ths I think, bolt for each one that you unscrew to get
at them. Check the exploded views in a moss catalog and you'll see what I
Subject: Gear box shifts out of gear
Date: Tuesday,January 09,1996 4:38PM
I have a minor problem with the gearbox of my '69 B: if I want to slow the
down by releasing the accelerator while engaged in third gear, it wants to
throw itself out of gear into neutral. Otherwise the gearbox is smooth. Any
ideas for possible causes and what it would take to fix it?
Frank van Dalen