1979 XS750 Transmission Surgery (and general overhaul)

meteorgnome

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This is going to be my ongoing project log, and where I gather all my questions. Thanks everyone for the help!


Last year, my transmission started giving me issues- Only got a few months of riding out of the summer before I had to take it off the road.

Symptoms started with difficulty shifting from neutral into first- it would "clunk' into first, as if it was dropping into it heavy

Than a few months later, after a 30 mile ride, I went to start my bike and heard a rattling sound that shouldn't be there- shifting into first was even more clunky than before, but I managed to get on the road- and tried shifting into 2nd and nothing but grinding/rattling- Not a good sound- Managed to quick shift into 3rd and was able to make it home- but that was it for the season.


Now it's time to get back on the road- Don't want to miss another summer!

I'm expecting to find maybe a bent shift fork and some damage to the gears when I finally get into the transmission.


Since I'm taking everything apart anyway, I'm planning on addressing as many things as I can- 'Replace everything that is necessary, and as little as possible'' is the idea- as my budget for this project isn't limitless


Currently, I've made it as far as getting the head off today, along with getting halfway there with the clutch.

Went to harbor freight, and Orielly's- and I ended up buying a 32mm deep Fwd Axle socket- But even that guy isn't long enough!



When I was working on the clutch, I found, sitting right on the bottom of the crankcase, a magnet- which I assume is supposed to be there- and, an 8mm hex bolt- sheared in half (both sides of it) - which definitely IS NOT. I haven't made it far enough yet to see where that bolt could have come from- it certainly wasn't anything that I did- I'm wondering if that has anything to do with the transmission issues I was having. We'll find out soon enough I expect.


So far, I found a few other things that might be issues- Would like to hear some perspectives-

#1. There's rust on some of my cam lobes. How big of an issue is this, and is there a way to take the rust off without replacing the whole cam? - And is taking the rust off in this situation something I want to do or am I looking at replacement? (photos 1+2)

#2. My clymer manual mentions, right before the 'remove the clutch damper assembly' -
removing 1, an outer small circlip, a flat washer ,and then a 'shim'

No shim on my bike. is this a problem? Partzilla has this shim listed, but it is of course 'unavailable'.

#3. Upon measuring my clutch nut, I discover the ... washer? that's directly under the nut is folded up on two opposite sides, as if hugging the nut. It doesn't look like it should be that way. It looks like getting a good grip on it with a deep socket (When i finally find one) is going to be a bit challenging because of it. (photos 3 and 4).

#4 . Rust on the flywheel. On the outer surface as well as some corrosion on the edges- Is this something I need to replace? Bike has been charging fine. (photos 5+6)

#5 Any idea on where I can find a deep socket long enough to hit that clutch hub nut? The shaft is 4 inches long. I tried harbor freight- bought a 32mm fwd axle nut socket at oreillys but even thats too short by an inch at least.
 

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The brown on the camshaft is lacquer from the oil. No need to do anything about it.

You might have more luck if you look for a long enough box spanner. They're like a tube formed into a hexagon at the end. Available in different lengths. At least that will get the nut off.

The rust on the rotor, don't worry about it.

I'll leave it for others to comment on the other stuff.
 
I will comment on some others I dont know.
#1 My opinion, they will be fine, polish the rust off if it is rust with some fine scotch bright.
#2 Not ideal try and locate a shim or make one if you can get the dimensions.
#3 That is as per the factory, Straighten the lock tabs with a punch, just enough so you can get the socket to fit nicely on the nut. It is a lock washer you wont be able to loosen the nut without the tabs being disengaged.
#4 Clean the rust off with scotch bright, then a light coat of oil. The XS758/850 has a brushless alternator, the metal rotor is magnetised from the flux of the energised coil . It will be fine.
#5 No idea.
 
Success!

Or partially at least.

Maybe I found the cause of my problems? Turns out the sheared off bolt is the one that secures the middle drive gear to the countershaft.

I THINK that it got sheared off in between the gears of the shaft levers- There's no visible damage to the levers- but I think they were misaligned- though I pulled the shift arm out a bit without knowing I should have been paying better attention to the initial 'are the dots aligned'.

Could this be the reason I "Lost' 2nd gear?
 
Both spring washer and 'plate washer' that are supposed to be there are missing- likely deeper in the transmission I assume.

Can't find the plate washer on any of the parts sites. Donno what to do about that

Yamaha OEM part number 90201-10595-00

https://www.partzilla.com/catalog/yamaha/motorcycle/1979/xs750sf/transmission

Part #30 is what fell out and got cut in half somewhere- 28 and 29 are missing - unless the magnet at the bottom of the clutch sump is the plate washer?
 

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Got the case split today. Big step forward.

Case-Split.jpg


Had bought a 'case splitter' tool off amazon- only to learn that that's really only for vertical split engines- so have another tool to return. Managed with a rubber mallet and some light pulling.


Going to get into a more thorough examination of the gears/forks tommorrow-

But- in the meantime- a question on bent shift fork diagnosis-

It looks like there might be a little bend in the first photo here- but looks like quite a bit in the 2nd photo. Am I correct here?

bent-shift-fork-1.jpg


bent-shift-fork-2.jpg




Also, looks like theres some oil seepage through the seal between starting motor and crankcase- Not too terrible- but obvious some build up right on the near edge of the starter. Not sure if I need to take the actual motor apart to check on that? Never had an issue with the electric starting before- motor seems strong,

 
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Made some major progress today.

Got to do some thorough inspection of the gearbox- andddd I certainly found some things to address

Bearing at the end of the mainshaft looks strange- corrosion. Seems to spin fine but....

Lots of rounding in geardogs and slots- Looks like I may need to replace 2nd and 5th gear on the mainshaft- 2nd being the worst.

And the shaft might be bent. I can't quite tell.

Then on the coutner shaft, 1st gear has some pretty major rounding- as well as the dogs on 3, and 5 could probably use replacing as well.

Then got into cleaning the bottom half of the case- purple power works pretty great. Sprayed on and scrubbed, then washed right off.

This is my first time into a transmission- and so I have nothing to go on- but take a look at the wear in the photos- tell me if my analysis is correct.


Now I have to find some gears. Anyone got any 750 gears lying around in good shape?

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host pics
 
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