I'm currently rebuilding my XT600e 1996 model due to a blown head gasket and a few other engine issues.
On the gearbox side of which I'm quite ignorant
The bike would occasionally jump from 1st gear to neutral when coasting down a steep incline and would occasionally jump from second to neutral under heavy acceleration. If you put it in second gear but didn't acccelerate hard initially it was fine.
The gearbox is pandoras box to me. I havn't split the case yet.
Is it worth delving into the gearbox or could there be something more obvious issue causing the bike to jump out of gear.

your advice would be appreciated.

hmmmmm doesnt look good mate.

As long as your sure that the not accidently knocking the selector then it could be a couple of things.

Without having to split your crankcases you can look at the actual shift mechanisn. Its very hard to explain what to look for if you're new to gearbox's , especially without pics.

The mechanism which holds the selector drum in gear can wear or break but this isnt as common as worn selector dogs on the gears. Once the gear has jumped, can you easily reselect it ???

If your a novice then I dont suggest that you get into rebuilding gearboxes yourself as its not very straight forward. By all means have a go but try and get help from someone who understands the design and principle (GET A MANUAL) or just remove the engine and take it to a shop.

Inspecting the selector mechanism should be straight forward though.

__________________
Did some trips.
Rode some bikes.
Fix them for a living.
Can't say anymore.

I think I know what you mean by selector mecanism.
Is it the multi lobed plate. (see attachment)
I did notice the finger that maintains pressure on the multi lobed plate had very poor spring tention on it to the point where when the finger was at it's lowest point it was in contact but only just.

To answer your other questions.
Once it jumps out of gear you can reselect it no probs.

I've been trying to find a soft copy of a manual.

Attached Thumbnails

 

Hi

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.

good luck!

Thankyou for your advice guys.
Looks like I'll have to split the case.
Now I have an idea of what to look for, at least I can make some decisions once I'm in there.
Best fire up the compressor and get the rattle gun out.
I'll keep you updated as to what I find.
I expect I'll be describing the situation and asking for further advice.

Look forward to hearing what you find. Take care not to lose any shims (little thin washers) that may be on the ends of the drive shafts and make sure you note where they go for reassembly. While you are in there have a look at 5th gear (the little one on the end)as this can start to fail on these engines - you'll see pitting on the faces of the teeth, maybe even with bits of metal or even part of a tooth missing. All parts that rub together should be shiney but not scored or damaged in any way.

worn dogs will be fairly clear too, look for rounded tops - there are more than one set so compare between different cogs to get an idea of what's normal and what's goosed.

It's easy when you're in there don't worry about pulling it to bits. Try and pull the whole cluster of gears/selector drum in one lump but no probs if it won't come out like this. Dump it on the bench and keep everything in the right order and orientation. Although it'll only go back together the right way it can be a bit of a struggle guessing what goes where if you just tip all the bits in a box and come back a week later to try and figure it out.