Topic: CB750 K7 pops out of 1st gear (video attached)

[vandehoorn]

Hi Guys,

I used this forum for quiet some while to find some more detailed information for my CB750K7.
However I never found the urge to start a topic myself until now.
But now I see to have a problem that not a lot of CB750 owners have.

Since a short while I found that my bike has a false first gear.
When I pop the bike into first gear it clicks and it feels like it is in first gear.
But when I want to drive away it takes off for a small amount and suddenly pops back to neutral.

This weekend I split the cases to have a further look on the transmission parts.
Thanks to the book of Hondaman I checked the gears and measured up the forks to see if everything is within specification.
Good news is that the gearbox looks pretty good to me and all measurements are far from the serviceable limits.

However I can find 2 issues.
1. The shifter arm has some play on the hinge which I need to solve according to Hondaman’s book.
2. I found that the 2 gears (C4 and C1 if I am correct) do not always engage properly.
I can imagine this is because I test the gearbox stationary but when I do lineup the dogs it does not spring into gear automatically.
I think this causes the issue that it feels like the bike is in first gear but actually it is not.

Please check the video below to see if you agree with me and please let me know where you guys think I can improve this setup.
Could it be the torsion spring or maybe the shifter drum?

All thoughts are welcome!

Cheers!

[maxheadflow]

With that issue, you might not have had to split the cases.   It's (IMO) an issue with the big roller detent (the round washer you pushed on) or the shift arm not having full travel.  It possible that the pin that that the shift arm return spring is setting on is bent.    If it does it all the time, you likey have rounded dog clutches and a bent shift fork.

[vandehoorn]

Hi Maxheadflow,

Thanks for your reply!

Yes I also think the "big roller detent" is the root cause of this issue since it just needs a small amount of force to snap the first gear into position.
I do not think the shifter arm is the problem at this point because once the dogs are alligned, the shift arm has sufficient travel to push the gears fully into place.

I made some detailed pictures of the shifter mechanism.
In my opinion I cannot find any parts that are damged or being bend.





Also I made some detailed photos of the C1 and C4 gear.
The dogs seem in pretty good shape if I compare them to the photos from Hondamans book:





I also measured up the shift fork length of the outer shift fork:
L-R top: 52.80mm
L-R bottom: 52,84mm

L-C top: 28,45mm
L-C bottom: 28,50mm

C-R top 30,57mm
C-R bottom: 30,41mm

So I think my shift arms are ok as well.

Please let me know if you see anything off on these photo's 
Seems the next step would be to get a new spring to see if this would solve the issue, do you agree?

Cheers!

[scottly]

Are all of the snap-rings seated in their grooves?

[69cb750]

The dogs and slots look good.
Check the pins the big roller centers on, check tension on big roller.

[maxheadflow]

1 thing to to note is that the tranny may hang up going into any gear when the motor is not running. You under stand that because you mention that the dogs won't mesh unless aligned properly.  Usually holding the one shaft and spinning the other so they align will allow the drum to rotate and drop into the next gear.  You show in the video that you have to add pressure to the detent for it to drop.  This observation may simply be OK. If the dogs are spaced correctly does it shift into first OK?   If not, it could mean that the detent arm spring is weak or the arm is simply hanging up. It could also mean that the shift drum, forks, gears are hanging up somewhere.   

Did it jump out of 1st whenever you got on it, or did it only happen occasionally?

[rotortiller]

Remove the springs, line up the dogs and turn the drum with a Philips screwdriver. Note the force and do the same from neutral to second. Compare and put one situation to rest or not. Then you know where to investigate. (Make sure the neutral switch is in place)

[vandehoorn]

Hi Guys,

Thank you for the comments!
It really helps that you guys help me to review the state of my tranny 

@Maxheadflow:
Yes once the dogs on the C1 and C4 gear are in line they engage properly without using excessive force.
Also I had the issue with the bike jumping out of 1st gear only very occasionally.

For example, it happened once when I wanted to drive off in a bend, from a standstill.
The motorcycle accelerated slightly and clicked from the first gear to neutral in the middle of the bend, causing me to drop the motorcycle 

I also found out that once I pull up the small roller, the big roller has enough power to push the gears into place.
So I think I just should start by replacing that torsion spring for the big detent roller:

@69cb750: There is some movement between the pins of the shift drum.
You can see that there is some space between the pins and the thin seperation plate.
However I do not know if this is too much or within spec?



And also the top disc on which te small roller sits on is not round at all, is this normal? 


@Rotortiller: Good suggestion, we do have a digital torque wrench at work which I could check this with.
Maybe I can do this tomorrow as an extra check.

@Scottly: I checked the snap rings on the shaft by turning them on the shaft and they all seem to be seated correctly

Thanks for the help so far!

[bamacb]

Check the shift drum for wear on the point that moves the fork . I have found this exact problem a few times

[maxheadflow]

Gotta picture of the first gear shift fork? 

It wouldn't surprise me if it's bad.

Need to see the side that slip into the sliding gear and the nub that runs in the shift drum.

[vandehoorn]

Hi Maxheadflow and Bamacb,

I think you are both refering to the same parts?

I made some pictures of the shift fork which pushes the 1st gear into place:


The shift fork looks as following:


The pin that moves inside the shiftdrum shows some signs of wear:




The slot inside the shiftdrum still looks ok to me:




I also measured up the fork fingers:
6,43mm
6,44mm

Wear limit is 6,1mm according to Hondaman's book

And I also measured up the pin:
6,90mm (smallest measurement)

Wear limit is 6,6mm according to Hondaman's book

So I should be ok right according these measurements?
Please let me know if you disagree or if you spot something 

Cheers!

[vandehoorn]

I saw I forgot to upload a picture of the shift fork on the side which pushes the first gear inwards.
Please let me know what you think 

[bryanj]

That looks a bit blue(overheated) on one side so personaly i would change it in case it is twisted

[maxheadflow]

Well I don't see any wrong with the shift fork or the dog clutch.   If they fork was bent, you'd see an edge of the pad scored where the fork runs on the sliding gear.  I'd suspect that it is something in the shifter.  It  would be worthwhile to assemble the shifter without the springs on the rollers and check to make sure that the bigger roller is well centered in its groove and it moves freely. Make sure the roller, rolls freely. Look at the cam the roller runs on. Try rotating the drum while making sure the dogs clear and try to feel if the drum rotation hangs up on anything.  I'd make sure that the rod the forks run on is straight also.  I'd look closer at drum at the arrow,  You may want to try an polish it  and the nub a little with some 600 grit. Not enough remove any grooves, just enough to clean the peaks.  I'd also look at the detent spring, possibly get a replacement.

It's my bet that it's not always going completely into gear.

[thealt]

I realize this is an old thread, but did you ever resolve this?
My bike has the same issue, but I'm not where I can do any troubleshooting.
Just curious if the spring was the issue since I like closure

Alt

[bacongrease]

  clutch adjustment ? ?