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'77 900SS won't stay in 4th gear

9K views 44 replies 14 participants last post by  Duccout 
#1 ·
Hi everyone,
New to me '77 900SS, first real ride on it today and found that it will not stay in 4th gear. Shifts into 4th just fine (either up from 3rd or down from 5th), but pops out of gear shortly after, even at light load or engine braking. All other gears work fine, and it shifts easily and cleanly into every gear. Any ideas or advice?
 
#33 ·
I was just watching Robax's first ride video after his rebuild and one statement caught my attention after reading this thread. In it he states that 3rd or 4th gear on these square case 900s is flawed and failure is common. VeeTwo has a replacement that is much better than the original. In fact VeeTwo has a full transmission if you are so inclined.

So, I still suspect the 4th gear dogs and not the selector box...

I hope Archilla will return here and let us know what's happening.
 
#34 ·
Slow progress. I've got other things demanding my time (mostly a newborn baby), so not much time to fiddle with the bike at all. I didn't expect a project when buying this bike, but oh well. Life goes on. I'll make it right eventually.

So far I can say a few things. There were some small issues in the selector box that I rectified. The selector wheel/shaft was not moving smoothly and was binding slightly in the bushing it rides in. I've also tried turning the selector drum directly via the stub end that comes out of the transmission and selecting gears while just turning the back wheel; every gear position, including 4th, can be selected positively and cleanly and remain engaged by directly turning the drum. Finally, I can reiterate that the stock linkage, upstream of the selector box, on these '76/'77 SSs is objectively terrible. The issue is not the crossover shaft itself or the number of connections in the linkage, but rather the specific connection between the shift lever and the actuator arm attached to the crossover rod. It's a sloppy sliding pin/slot interface with a lot of friction and freeplay; it's an inefficient and imprecise design when the parts are new, it only gets worse when the parts are worn. It's hard to appreciate how bad this detail of the linkage is unless you've seen the specific execution on one of these crossover shift Super Sports in person. Anyway, this sloppy connection between the shift lever and the rest of the linkage is such a fundamental part of whether or not a solid and accurate input is made to the selector box (and subsequently the shift drum) that I don't think it is worth proceeding until I address it. Converting to RH shift would be one idea as others have suggested, but I don't believe it needs to be that drastic; the various parts of the linkage just need to be properly connected with real rotating joints (ball end heim joints). So, I'll be doing some fabrication so that I can fit rearset pegs/levers and connect the shift lever to the crossover shaft via a proper linkage rod with balljoints at either end (similar to the stock '78+ SS peg/lever arrangement). Once I'm sure the linkage is precise and I know the transmission is getting good, accurate inputs, then I'll reassess the shifting on the road.

@wdietz186 Your description of the root grooves of the gear teeth being machined right down to the engagement dogs is clear and makes a lot of sense as a weak point, but I only see that happening on 5th gear (on the layshaft). On all of the other gears in the box, it seems there is plenty of radial clearance between the outer diameter of the engagement dogs and the root of the gear teeth. If there is a specific structural weakness with the dogs on 3rd or 4th gears, what is it? Having not seen the parts in person, I certainly could be missing something, but by looking at photos of parts (individual gears and complete gearbox assemblies), I can't see anything that stands out about the design or machining of 3rd/4th that looks uniquely problematic.

-R
 
#35 ·
I go back to Cook Neilsen, who won the Daytona Super Bike Race on one of these bikes, but during the course of development had to fit new fourth gears for every race, and his bike was right-hand shift. In order to keep racing he had a new, complete gearbox made in the USA, copied exactly from the original, by Marvin Webster, who stated that there was nothing wrong with the design, but the problem lay in the fact that the gears are made of soft gear steel, and then case-hardened, which is OK for a low torque engine, but not for high torque, which is why the 750 Ducati had no problems with the same gearbox, but the 900 did. Basically, the hardening on the dogs wears, allowing the corners of the dogs to round off and then to slip out of gear under load.

To put this in perspective, when Suzuki introduced the bullet-proof GS750, it used a case-hardened gearbox, but when the GS1000 came along it had a gearbox made of steel that was hard all the way through.
 
#36 · (Edited)
Hey guys, it’s been a while, so time for an update. The photos below tell the story. After methodically eliminating all possible problems external to the trans, it became clear the issue is indeed in the trans itself (as several of you suggested from the beginning). Soooo, I removed and crated the engine at the beginning of the year and it’s off to have the cases split and the trans fixed. I’ll report back again when the engine is back and the bike is together.

Through the whole experience so far, I accomplished a few things… First, I designed and made my own rearset brackets to replace the stock pegs and use Tarozzi pegs/controls (pics below), with all connections using metal/metal heim joints (including replacing the clevis pins between the crossover shaft and the shift box). This setup gives super solid and precise shift feel, and I guess is at least as good an earlier (’75 and earlier) right hand shift, or later (’78 and later) integrated shift setup. If anyone here has a crossover shift SS (’76-’77) and wants more information or details on my setup, let me know. Second, I now have a pretty good idea of how to secure and crate these engines. The crate I made is built like a tank; it can probably be reused several times, so if anyone in the SoCal area needs to ship their bevel twin engine and wants to borrow the crate after my engine comes back, let me know. I can also give advice on dimensions and materials I used for mine, etc.

-R

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#40 ·
Nice set up with the gear and brake linkages, I have a crossover shift SS (’76-’77) and would like to know more details about your set-up. I am one part away (0755.13.020) from returning my bike to original spec and if you have this part and feel you could pass it on to me, I'd like to try this first before I invest in a solution like yours .linkage
 
#41 ·
Hi all,

Sorry for disappearing for a few years... Time flies. Anyway, short version: 4th and 5th gears on the mainshaft were replaced when the cases were opened, as was one shift fork (which was completely knackered). With everything back together, and with the custom rearsets and linkages that I made (using metal/metal heim joints at each connection), the shift feel is super precise and direct, and it shifted cleanly into every gear. I did notice sometimes reluctant returning of the lever after downshifts on my shakedown ride, so I replaced the return spring in the selector box (even though the one that came out looked fine) and that problem was solved. So now the transmission is solid, and the shift feeling is properly good. Below is a pic of it all together.

That's the good news. The less than good news is that about a year ago, someone who was not paying attention ran into it and knocked it over while parked in a very quiet lot, parked in an end space, and with me sitting nearby, within sight, drinking a coffee. This is, no joke, the only time I had ever parked it anywhere other than at my house. I was furious. But, life goes on. I've been busy since then, so chasing down replacements for the damaged parts to make it right, and working with a painter to repair and refinish the fairing and fender, has taken me since then, so about a year. I have almost everything needed in the garage now, so with any luck, I'll have it back together again soon.

@wildgg I'll reply to your questions separately via DM.

Tire Wheel Fuel tank Automotive fuel system Vehicle
 
#42 ·
Thanks for the update, what a beautiful bike. The problem of idiots damaging our pride and joy's is a pain, but something that we have to live with, and I often repeat to myself a phrase (with a slight modification) from Cycle magazine: 'you put that bike on the road boy, and it ain't worth nothin'. Hope you get it repaired to your satisfaction soon.
 
#43 ·
I share your pain. A few years back, I left my SSD in a car park in Urbino and came back to find the faring scrapped and scratched and the screen smashed. I can only assume someone had tried to move it to make space for his/her car and had dropped it. Wonderful ...

Anyways as Colin says, that's the risk we all assume (unless you want to own a dead 'museum piece'). Enjoy putting it back together and glad you got the shifting issue sorted. Sweet.
 
#45 ·
I also have a tale to tell: I came back to my 750 Sport in a car park one day, started it up and pushed it off the stand and went to pull in the clutch, and there was no clutch lever! It was broken off and laying on the floor.... A guy came out of a shop and told me that a car had reversed into my bike and driven off and he had picked it up for me.
 
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