True to form our 1978 XS750 seems to pop out of 2nd gear and it seems common with the Yamaha triple engines. After looking around we found a few forum posts on fixing this but nothing really with pictures and more detail.
So far we’ve removed the shim/ spacer between 2nd and 5th gear but need to repair the gasket seals before testing to see if this helps the gears mesh together. We’ve really just been focusing on getting access to the 2nd gear on the lay shaft and I’ll update this post if we have to do anything else to resolve this problem.
The work itself is easy and you’ll probably want someone to help you but its not necessary it just makes things a bit easier. You’re going to have to be quite forceful in getting the bolts and casing off along with removing the 2nd gear on the lay shaft, but we managed to do this with basic tools and alot of straining.
First a diagram from the manual of what we’re after and then why…
The reason why the gears slip is wear between the 2nd and 5th gear pinions on the lay shaft, the 5th gear has dogs/ studs which connect and lock into the second gear, if these studs are worn or rounded or the slots in the 2nd gear are worn this will allow the gears to slip out. One fix for this is to move the gears closer together by removing a spacer to stop them slipping which is what I’ve started looking at.
You’ll need hex keys, a mallet, some emery cloth and a few spanners – mainly to get extra leverage on the hex bolts, unless you’ve got decent ratchet handles etc… You may also need some silicon gasket sealant or new gaskets if like mine your’s have torn.
Get the bike up on a lift or paddock stands and drain the oil before proceeding… you may also want to remove the exhaust section if using a lift.
On the left hand side of the bike we need to remove the casing. First remove the clutch cable, gear shifter and foot peg.
Remove the 3 bolts for the timing case cover and remove the cover – its the bit with Yamaha written on it. Inside there are 3 screws that hold on the timing plate, you need to mark their positions – take a photo and use a knife to scratch in a mark for each one and remove each one. Next remove the center bolt, hold the outer nut (gold) and loosen the center bolt (silver) to remove it.
Now thats done you can either completely remove the timing plate or just let it hang from the wire. Now we can work around the rest of the casing removing the bolts. Start at one point and work clockwise around keeping the screws in order of removal so you can make sure you replace them as you found them.
If its been a while since this casing was removed its going to be really stiff, time to call in Senior Tappy! Basically get something to protect the casing, e.g folded tea towel etc.. and with that between the casing and a hammer/mallet hit it a few times to loosen the seal. The casing then should come loose. Remove the casing, watch out for spindles and cogs – there’ll only be one loose.
Now we can remove the layshaft bearing housing cover which will give us access to the area we need to work on. There are 4 screws to this cover and it sits behind the gear shift lever. Its going to be really stiff to remove so you’ll need Senior Tappy again to shake/tap it loose. It will take a lot of work but persist it will free up just keep wriggling it to get it over the gear shift lever.
Ok, now if you look inside you should see a cog. That’s 2nd gear on the layshaft which apparently is what causes the issues of the gears popping. So we’ll remove the circlip and shim in front of the cog, for the circlip some needlenose pliers should do the job.
Here’s the worst bit – removing the cog. Now you can take the sump cover from the bottom of the bike but its a hassle and as long as we’re only dealing with 2nd gear here we can get away with out needing to do this. First get some emery cloth and lightly sand down the tops of the 6 ridges that were in front of the shim and circlip. We only want to remove the dirt and crud there so the cog will slip off. To get this cog in and out you’re going to need brute force and a few small hex keys. You should be able to get the short length of the hex key behind the cog by sliding it down the left hand side of the gear. With this in place get some pliers and get a really good grip on the hex key and begin to pull the cog out – it should give. It will take some time but keep at it – best to have 2 people taking turns. If you take the sump cover off you can probably push the gear off rather than pull it this way but its more effort really and my XS750 doesn’t have a centre stand and its on a jack.
Remove the shim that was behind the gear pinion – it may be attached to the rear of the gear. Now reassemble to gears and casing leaving out this shim. With this spacer gone 2nd and 5th gear should lock together better and prevent the gears slipping out, you may want to add the shim you removed to the front of the pinion to help push it closer to the 5th.
Thats it! (I hope)…
Now if all of this doesn’t work there are a couple of extra options, you can get replacement gears off of the XS650 as its chain driven and the pinions won’t be so worn, or get new replacement pattern parts. You can also machine your existing gears to remove the rounding on the dogs of the 5th gear to get a better fit to help stop slipping. Also you can look at the shift fork on the right hand side of the bike.
I’ll update this post if I have to do anything else to remedy the problem.
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