Gearbox Problems

[bankrobber]

hi looking for help, cant get 4th or 5th gear on my sherpa 91 model, will put into neutral go down to first ok back up 2nd and 3rd ok but lever wont get back any more sometimes goes into neutral where 4th should be really stumped also can put into 1st and 2nd not using clutch, changed gear oil no help, thinking may have to take casing off will i need any special tools and what to look for when casing off, also finding neutral is a nightmare kind regards in advance

[0007]

Did this happen suddenly? Like right after hitting a rock on the shifter?

If the shifter sticks when the shifter shaft is bent, and usually the shift shaft comes out the clutch side

If a shift drum or fork were damaged it would effect shifting but it wouldn't make the shifter stick

So....shift shaft is my diagnosis

[woody]

It's possible to assemble the gearbox with one of the selectors upside down, or wrong way around, forget which. If this happens then you get no 4th or 5th gear, only 1, 2, 3. So if the engine has been rebuilt this is a possible cause.

If this has happened since you've been using the bike with all gears working, then you could have a broken or bent selector.

As per the question above, when did it happen, after a rebuild or during use.

[bankrobber]

thanks for replies when i bought the bike it was advertised has having a engine rebuild so maybe woody could be right, since buying bike only played around in garden so never used higher gears till i took it out on road the other day the shifter does not look bent so what do i need to do now thanks

[woody]

If the shaft was bent and causing a problem, the shafts movement throughout the entire range would most likely be affected, not just prevent 4th and 5th engagement. Easy enough to tell if the shaft is bent by removing the clutch cover and seeing how easy it is to withdraw it over the shaft. If it's bent it will most likely come out with the cover, rather than stay in situ with the cover sliding easily off it.

Once you've checked the shaft, remove the selector pawl assembly that the selector shaft engages with (three screws) and check that the two pawls haven't got chipped or broken edges which will stop them engaging properly in the selector drum. Unlikely as again, this would affect movement throughout the entire range, not just 4th and 5th gears.

Once you've done that, there is nothing left to check really that isn't inside the gearbox. Maybe if someone has assembled the engine clumsily when joining the cases, the selector drum housing could have picked up and made it stiff, but again you'd expect that to present itself in all gears.

As it's had a rebuild it does seem the most likely cause may be the selector fitted incorrectly, which means a strip down to correct the fault. I'm sure the Haynes manual may even refer to this but been a long time since I looked in one. The selector fork in question has bosses either side where the shaft runs through. One is larger than the other. If the fork is the wrong way around, it can't slide far enough to engage 4th gear.

The last time I experienced this was in 1977 on a mate's Pursang that he rebuilt and got the selector wrong, so it's stretching my memory a bit... We took it out to try it after the rebuild and only 3 gears.

[bankrobber]

Hi back again still not sorted took engine out gave to mechanic found a broken spring replaced put back together took four weeks still not working, I think the selector problem is best bet can I do this with engine in or will I have to take back out, how will I know which selector is the wrong way round, will this job be too much am ok at basic stuff but never stripped a engine before will take photos as going along any more tips many thanks in advance.

[bondy]

you can take the selector arm off why its still in the frame but you cant take the selector drum out you have to spilt the engin for that im afraid.

Edited May 12, 2013 by bondy

[woody]

The only spring is on the selector mechanism inside the clutch cover and a broken spring on that won't cause the problem you have. The selector moves backwards and forwards when you move the gear lever and it is moving unhindered if you can go through gears 1 - 3. All a broken spring will generally do is stop the gear lever centering after you change up or down. At worst it could foul and jam the selector but then you wouldn't have gears 1 - 3, so not sure what your mechanic has found broken.

With the clutch cover off, you can see the selector that the gear lever operates. This engages with the selector mechanism which in turn engages with the selector drum which moves the selectors in the gearbox which select the gears.

Picture 1 below shows the selector mechanism in situ which is held in place by 3 screws. The selector that the gear lever operates is removed and can be seen on the stand. It's been turned around to show the spring, when fitted the spring faces inwards. This is the only spring I can think of that could have been replaced.

The slot in the selector fits over pin that you can see at 12 O'clock on the selector mechanism. When you move the gear lever up or down, the selector moves backwards and forwards. Behind the selector mechanism are two pawls and these engage into slots in the selector drum. When the selector is moved, the pawls engage with the slots and rotate the selector drum inside the gearbox. This moves the selectors inside the gearbox which engages the gears.

If you remove the selector mechanism by removing the 3 screws you will see the outside face of the selector drum,an example of which is see in picture 2. If one of the slots is damaged it could possibly stop 4th gear from being engaged as the pawl would not be able to locate into it to move the drum. I've never seen it happen and think it's unlikely.

Most likely I still feel, is that the previous owner reassembled the selector fork the wrong way around when rebuilding the engine. If all the mechanic did was replace the selector spring, he hasn't had to split the engine to do so, therefore the selector forks haven't been checked. All he had to do was go through the gears when he'd finished whatever he did to check they worked - which he obviously hasn't

The selector forks are shown in picture 3 and the one nearest is the one which can be fitted the wrong way around. It's correct in the picture with the larger boss to the outside. Picture 4 shows them in situ in the gearbox

I can't think of anything other than a selection problem inside the gearbox that realistically is causing the problem and I'm fairly certain it could be a wrongly fitted fork. I can't guarantee it but there is only one way to find out. For someone used to these engines it is less than 1 hour to get inside them to investigate

Edited May 13, 2013 by woody