Topic: Transmission Troubles - Second gear stutters and kicks

[Stevenarrow]

Hello,

I am having issues with my cb750 K5. When accelerating in 2nd gear the bike stutters, kicks and loses power. The issue only happens in 2nd gear between 30-40 mph. All other gears and speed are fine: clutch, timing, and carbs are fine.

Working with my mechanic, he assures me, it is the gear selector forks. My mechanic is great and loves to educate; i was hoping that someone with specific knowledge of this bike / issue can confirm his suspicions. Reading more on this issue in the forums; it seems not uncommon for his assessment to be true.

If this is the issue, then splitting the case is the only way to fix it. i could do this with my mechanic's help but it will cost me time: any advice.


thanks for looking.

[750essess]

your mechanic is correct. just repaired one like this the shift fork will be bent and most likely the gear dogs will be worn for that gear. splitting the case is the only way to repair.

[Stevenarrow]

Thanks for the reply -- it's good to hear from people who have experience with the machine in question.

Should i then be prepared to replace both the selector fork and the gear itself for the repair? Are there any other parts i should have on hand before i pull the engine?

[MCRider]

CB750s would damage 2nd gear usually from abuse by the owner. The symptom is usually that it jumps out of second gear into a false neutral. Especially when accelerating.  The engine would then over rev, and the frightened bunny would try to cram it into gear again, furthering the damage.

The 1st to 2nd shift is the one where jack rabbits are trying to shift fast and really put their foot into the process, bending the shift fork and rounding off the edges of the engagement dogs on the respective gears.

But your symptom description is puzzling. Does it actually lose power, or does it jump out of gear?

[Stevenarrow]

But your symptom description is puzzling. Does it actually lose power, or does it jump out of gear?

Sorry, I don't mean that it loses power from lack of combustion or anything -- I mean, loses power from lack of gear contact. The symptom is only brief and happens when under acceleration in 2nd. It does not jump out of gear.

[MCRider]

But your symptom description is puzzling. Does it actually lose power, or does it jump out of gear?

Sorry, I don't mean that it loses power from lack of combustion or anything -- I mean, loses power from lack of gear contact. The symptom is only brief and happens when under acceleration in 2nd. It does not jump out of gear.

OK that's probably it. I've never heard of one that didn't jump out of gear and STAY out of gear. "Brief" as you say. Not much else it could be though.

[Stevenarrow]

Thanks,

I guess my next step is to track down the parts.

[MCRider]

Thanks,

I guess my next step is to track down the parts.

Potentially more bad news. I think the center fork (the one you need I think) is discontinued, though you may find a dealer with NOS. The gear is also, but Yamiya has replicas for $450+ I think. They may have repro forks now too.

Good used is best bet, but a gamble.

[Stevenarrow]

thanks for the heads-up.

Is there any good reference material as to which parts are interchangeable with other K model 750's...?

..will a K4 or later model fork and gear fit into my transmission?

[MCRider]

All the Ks, at least up to 77/78, K7/K8 are interchangeable. The late ones may be also. I would bet, I just don't know. Same for the Fs, I think.

[Stevenarrow]

good news. there seem to be plenty to choose from on ebay.

[MCRider]

I see Yamiya now has the shift fork as well... for $262. Anything less than that is bonus.

http://www.yamiya750.com/index.php?main_page=index&cPath=21_64

Assuming you didn't know, yamiya is famous for finding sources to reproduce unavailable parts. Often the prices are pretty high.

You may find a friendly dealer who can do a parts search and find a leftover gear and fork at a dealer or warehouse. The original Honda OEM prices were about $130 and $60 IIRC.

[Stevenarrow]

thanks for the link. But i think i have to check the used market before committing to the higher prices.

...I just got a complete transmission from another K5 with 11000 miles for $40. I'll check out the condition and go from there.

Thanks again for the help.

[MCRider]

thanks for the link. But i think i have to check the used market before committing to the higher prices.

...I just got a complete transmission from another K5 with 11000 miles for $40. I'll check out the condition and go from there.

Thanks again for the help.

Wow! That's great, should work. Damage to the fork and gear are usually obvious to the naked eye.