Stopper arm spring broken. Allows arm to float, causing sporadic gearchange operation. Replace spring (Chapter 2).
Gearchange mechanism selector arm broken or worn, or missing pins on selector drum. Remove the gearchange mechanism and check the arm and all components (see Chapter 2).
Jumps out of gear
Selector fork(s) or selector drum tracks worn or damaged. Overhaul the transmission (Chapter 2).
Gear groove(s) worn. Overhaul the transmission (Chapter 2).
Gear dogs or dog slots worn or damaged. The gears should be Inspected and replaced. No attempt should be made to service the worn parts.