Topic: How to improve shifting CB750 smoothness and fork/gear movement??

[mycb750k6]

When I look at this youtube video, I assume the shifting should be as smooth as this example. I find turning the shift drum on my early CB750 K model  by hand and changing gears in this way with an open case to be stiff and stiff shift fork movement. What needs to be done to make this shifting mechanism smooth like the video? I've tried a number of gear sets with drums and forks all with some amount to dog wear but I don't see what is causing the stiffness when I turn the drum. It seems to be stiff when  the gears slide or when the forks move and the sliding of the forks when the shift drum is turned. Any ideas on how to make this all work smoothly?? Sometimes turning the shift drum wants to push the drum out of the case rather than move the fork or gear.


CB750K7 Transmission Set up & Test

[JimM68]

When its running, it wil all ove smoothly.
Ya just can't test this unless the motor is running and the bike is moving.

Sent from my ADR6325 using Tapatalk

[mycb750k6]

I appreciate that but the video seemed a lot smoother than mine and since I am chasing stiff shifting I'd hoped to be able to verify my fix.

[dave500]

your not playing the right soothing music in the background.

[mcpuffett]

Dont forget all his gears etc are sat in a open lower case so there is no resistance there  , cheers Mick.

[mycb750k6]

Mine is in an open case just like that. And I tried the soothing and other kinds on music but no joy

[Jerry Rxman Griffin aka MuthaF'er]

Secret lubrication and fresh bearings? Well broken in together?

[Danno]

about the only thing I know tht makes these shift any better is if you have undercut gears which stock does not
 also new shift drum and forks helps too

[HondaMan]

There are several spots to check:
1. Shift pins on the inside of all 3 shift forks. If one is rough, it drags on the drum.
2. Slide movement of the gears. They must slide as smooth as a baby's bottom.
3. Condition of the grooves in the shift drum, especially where the shift fork pin is being pushed sideways: sometimes these spots develop a sharp edge right where the beveled opening (toward the OD of the drum) makes the initial turn. This can cause drag.
4. The shift drum's holes in the case. If the outer one is worn oval, the inner one tends to bind. If the drum binds just before it starts to push out of the case when shifting it by hand, this might be the culprit. The Neutral switch/detent pushes upward pretty stiffly, wearing the top of the hole if light oils were used for lots of miles. This makes the hole oval toward the top, and this can be measured with narrow feeler gages. This one can be expensive to fix, requiring a new bushing in the case for the drum.

[MRieck]

I have polished the sift drum channels, shift fork pegs and the shift fork shaft on different bikes. Casadium coating would work well too.

[MCRider]

MyCB: as far as the video goes, I think he has a knack for turning each item at just the right time for engagement.

Do you have the clutch basket on yours? That weight might give a different feel. Without it as in the video, maybe that lends the appearance of easy shifting?

Wish I'd done that while i had mine apart, just to see everything moving around.

[mycb750k6]

I don't know, the video sure is smooth but the drum channels and other things look polished too. I'll try that.

And the drum binds just before it starts to push out of the case when shifting it by hand   so I'll check that far hole measurement too.


Frankly there seems to be nothing to prevent the drum from pushing out when the lateral load on the fork from the angled channel in the drum  is applied - and then it binds.

[Pecantree]

MyCB:
Another reason that video shifts smoothly is he is applying pulsing power with his fingers. So the gears have a back and forth slop that gives the drum and forks little nudges to shift smoothly.
I've been playing with this on my case and have set up the same thing. Also installed all the external shifting mechanism.
Shifting while spinning by hand was very smooth.
Then slipped a piece of hose on the main shaft and rigged my cordless drill to that.
Shifting was a little more problematic with a constant load.
Here's a <a href="

to that video, my weird noise starts at 1 min in.

The dogs are hitting, I think it's a combination of gear bushing slop and slightly bent forks.
How do you measure for a bent fork if you don't know if any are good?

The link to that post is here.
Good luck, I'll be following your fix.
Steve

[HondaMan]

I have polished the sift drum channels, shift fork pegs and the shift fork shaft on different bikes. Casadium coating would work well too.

Can you elaborate on the Casadium, M?

[mycb750k6]

Nice video. I'll try the drill too for spinning the gears after polishing all the parts up. Not sure yet how to polish hardened steel. That sound at the one minute in spot sounds like a dog but who knows. Does your drum try to push out of the case at all when shifting ? I still have a ways to go to even get it as smooth as yours.


BTW, is there a picture of back cut or undercut gear dogs anywhere here?

[Pecantree]

Let me know how you end up polishing parts.
The shift drum does try to push out. I installed the neutral detent switch to hold it in place.
Yeah, Its the Countershaft fourth gear dogs hitting and the mainshaft 2/3rd gear dogs hitting the mainshaft 4th gear.
Still trying to figure out why.

[HondaMan]

Let me know how you end up polishing parts.
The shift drum does try to push out. I installed the neutral detent switch to hold it in place.
Yeah, Its the Countershaft fourth gear dogs hitting and the mainshaft 2/3rd gear dogs hitting the mainshaft 4th gear.
Still trying to figure out why.

Make sure the 4th gear is in the right way: it's easy to have it in backward.

[Hinomaru]

Speaking of K shift drums ...

Most of you know that Honda introduced a modified shift drum beginning with K4 engine number: CB750E-2304501. Owners of the earlier K models *can* take advantage of this modified design change for improved gear engagement.

In case you missed it, here’s the Honda service bulletin: (PDF format)

http://data.sohc4.net/SB750/750_40.pdf

[Danno]

use 2000 grit sandpaper to polish it will look like chrome you can use a strip taped to the flat  end of a drill bit but you must go very slow and not much pressure you want to polish not cut

you can find that paper at most automotive finish stores