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78 GS750 Shifting problem After engine rebuild

  • Thread starter Thread starter ptakattack
  • Start date Start date
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ptakattack

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Hello, I am just finishing rebuilding my 1978 GS750. The entire engine has been disassembled, including splitting the crank case.

The bike is back together now, however I am having shifting problems. The first time i started it, the bike was getting caught up on something (it wasn't running long). I pulled the oil cover off and noticed one of the shifting forks was out of place.

Once that issue was taken care of, I was only able to get the bike into 1st and neutral. After searching around the forum, I realized that the shift shaft was one tooth off. I moved it to the correct place and now I have a false neutral, gear 3, 4 & 5, but it will not go into 1, n or 2.

Does anyone know what could be causing the issue? My only thought is that the shift fork may have bent when it was misaligned. It shifted through all gears easily before I pulled it apart.

I've double and triple checked that all of the forks are in the correct position and the cam stopper is resting in the correct place.

IMG_2181.jpg
 
I was able to play around with it some more yesterday. I think the problem is more involved than just the prawl being out of alignment. I ordered a new set of shifting rods and forks. I'm going to swap them out on Sunday and see if that fixes the problem.
 
Thanks Chuck, I have the manual printed, and I've compared it closely. I ordered a set of shifting forks and rods for a total of $15, which should come in this evening. I'll mess around with it some more before trying to swap out parts. I'm hoping to have the problem solved some time tomorrow (we'll see if that actually happens though).
 
Latest update:

I swapped out the shifting rods and forks. The forks would not move smoothly on one of the rods (the one with the two shifting forks attached to it).

That may have been contributing to the shifting problems, however, it is still feeling the same as before I swapped out everything.

I'm hesitant to put everything back together. As it stands now, I can have 1st and N, or if I manually turn the shifting barrel, it will go into 3rd, 4th, and 5th. This is all sitting still, but I have it on the center stand and I've been turning the rear wheel and the clutch shaft and it is not helping it shift.

My question is, should I put everything back together and hope that the problem resolves once it's started and has oil circulating, or am I overlooking something? I've drained the oil 4 or 5 times already and I getting tired of disassembling and reassembling everything.
 
Slop oil on it before joining the cases..Im assuming they are split?? Anyway, be sure the bearings are seated squarely on the ends of the drum.
 
I actually was able to do it without splitting the case, however, I did take out the engine, split the case and made sure everything was seated properly about a week ago, which did not solve the problem.
 
I think the next step is going to be reassemble, add oil let the bike run for a little while and see if shifting returns to normal. If it doesn't, I'm going to order a different transmission on eBay and just completely swap them out.

It's so close to being finished, if it would shift properly, all I would need to do is basic adjustments such as syncing the carbs and timing adjustments. I'm pretty frustrated with how long this is taking me to solve the problem.
 
ptakattack said:
Latest update:

I swapped out the shifting rods and forks. The forks would not move smoothly on one of the rods (the one with the two shifting forks attached to it).

That may have been contributing to the shifting problems, however, it is still feeling the same as before I swapped out everything.

I'm hesitant to put everything back together. As it stands now, I can have 1st and N, or if I manually turn the shifting barrel, it will go into 3rd, 4th, and 5th. This is all sitting still, but I have it on the center stand and I've been turning the rear wheel and the clutch shaft and it is not helping it shift.

My question is, should I put everything back together and hope that the problem resolves once it's started and has oil circulating, or am I overlooking something? I've drained the oil 4 or 5 times already and I getting tired of disassembling and reassembling everything.
Click to expand...
This seems to be the source of the problem. Did you have the same issue with the original shift rod and forks? I'd try oiling the rod and fork holes to see if you could get the forks to slide better.
 
On the shift shaft (clutch side) the toothed plate has a window in it. When you install the shaft, you center the spring ends (in this window) over what looks like a smooth stud. This stud, might call it a pin, is adjustable. You never ever loosen this for any reason. It will cause the shifter shaft and the pawl assembly to not line up correctly.....Did you loosen or take it off???.....
 
I think I know what you are talking about, and it was removed. The entire engine was disassembled (minus taking the gears or the crankshaft apart). That is good to know that it is adjustable, I will play around with it tomorrow.

Hannibal:
The forks were not moving easily on the original rod that were installed, they move smoothly now that I have replaced the rod.

I haven't touched the bike today, so the oil pan is still off the bike and the clutch is removed. The engine was completly apart for about 3 months, I think it's possible that some of the moving parts on the transmission became bound up while it was apart.

This is my first time rebuilding an engine, so some of the things that a more experienced mechanic would have checked were not done prior to assembly (such as making sure all the gears on the transmission move as they are suppose to).
 
Post a pic of the shift shaft/pawl alignment with the trans in neutral.
 
This video is a few days old (before I swapped out a shifting rod) but still relevant because it is still doing the same thing.

in the video it is clicking down to 1st gear and up to N, but it will not go beyond N.

https://drive.google.com/open?id=1t5PaEizdNtsNYjfrCGYUn6prS-LegfE2

also, in the video, I am not turning the wheel or the clutch shaft. Even with turning the wheel and clutch shaft, it will not go above N.
 
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Not sure this is relevent, but does the shift drum have a double hooked spring that attaches to the case webbing?
It's a lightweight spring maybe 2" long with longish hooks on both ends.

It's part #17 in the gear shifting oem part diagram.
I call it the shift detent spring.
 
It for sure has to ratchet the drum between each gear. Thing is its been so long since ive been in a trans that I don't remember the specifics.
 
The spring is attached. The GS750 manual wasn?t super clear with the positioning of the spring, but I compared it with a gs1000 manual and I?m fairly certain it?s on correct
 
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