The gears can jump out for a couple of reasons:
1) they haven't been fully selected properly (in other words there's not been enough movement between your foot and the selector forks (6,7,8) that move the gears. The selector forks are moved by the selector drum but if they don't quite shove the gears together like they should, the spring detent (the springy thing -11,12,13 above) won't quite get to it's stop on the plate to hold everything in place. Causes can be a loose gear lever on the shaft or wear in the mechanism that rotates the selector drum. Worn selector forks can also cause this as again they can't fully shift the gear home. The manual may have a figure for fork thickness you could measure. Sometimes the forks can get bent too which would in turn cause shifting problems.
The detent mechanism should press quite firmly on the plate.
2) as tedmagnum says, you could have worn dogs (woof). As you shift gears, ones the same shaft often slide into each other so that dogs (small square shaped lobes of metal) on the face of one gear slide into holes on the face of it's mate. Once the dogs are engaged they transmit the drive. If the gearbox has been abused, such as by receiving rough clutchless shifts, the poor dogs can get worn and rounded so that especially under load they can slip out of the holes and hence you jump out of gear.
no way of spotting this without a stripdown. Rounded dogs can be ground flat but it has to be done to exactly to the same extent on each one otherwise the largest remaining dog will take all the load. Worn holes need the whole gear replacing.
Last edited by rickg; 26 Apr 2007 at 20:50.