Topic: Bike pops out of first gear on hard acceleration

[Jonm99]

Hi guys,

I'm about 1k km after a full rebuild of my bike, all but the transmission and clutch that is...

Bike started popping out of 1st on hard acceleration and seems to be getting progressively worse (happens on not so hard acceleration now). All other gears work fine and shift smoothly, including neutral. First was fine before and after the rebuild.

I was using 10w 40 semi synthetic oil, only to read here that it can cause clutch issues. Not sure if that's related, but I switched to 20w 50 synthetic (wet clutch) oil, with no noticeble change. At first I thought it was a clutch issue, but it now seems (to me) to be a problem with the transmission and/or related components.

I understand that it could be rounded dogs on the gears, bent or worn shift fork, worn bearings, worn shift drum, or maybe just loose pins on the end of the shift drum.

Can someone please walk me through the process of identifying the issue, from easy to check / fix to I really don't want to tear down the engine again?

Thank you,
Jonathan

 

[samm_j2]

I have herd it suggested to not use synthetic oil on these older bikes. I know that is speculative at best...

Curious to hear what suggestions people have before tearing into the tranny...

[bryanj]

WHAT BIKE

[goodtryer]

1977 cb750k with original airbox, slow jets=35, main jets=120, Delkevic 4in1 headers, 18" Cone engineering QC muffler, 7" headlight, dynatek ignition, ss r/r, clip ons, and ebc front brake rotor. All the rest stock, for now. First restoration, lots to learn!

[bryanj]

Dont see that info on this post and i for one dont have time to go reasurching to find out

[TwoTired]

Dont see that info on this post and i for one dont have time to go reasurching to find out

Hi Bryan!
Did you happen to look at his sig file at the bottom of his post?  :-)

But to me, it seems that worn or bent shift forks.  Worn/loose shift drum, worn shift dogs, or shift drum pins.

Might be a short shifting foot issue.  Or, someone experimented with no clutch shifting.

Can't believe it's a synthetic oil issue.

Cheers,

[bryanj]

Never look at sig, didnt think i needed to as its only polite to say what you are asking about. As to 750, jumping gears is normally down to bad gears and carying on using it only makes the dogs worse

[Jonm99]

Thanks guys.
Which of the above (worn or bent shift forks.  Worn/loose shift drum, worn shift dogs, or shift drum pins.) can be inspected (and replaced) without pulling the engine out? Shift drum and / or shift drum pins? Or just the pins?

And if none, how do I access the gears, from the bottom of the case or do I need to go in from the top and take the whole thing apart?

Thanks again,
Jonathan

[newday777]

Access the trans from the bottom yes.

http://forums.sohc4.net/index.php?topic=105038.0

Dont see that info on this post and i for one dont have time to go reasurching to find out

Hi Bryan!
Did you happen to look at his sig file at the bottom of his post?  :-)

But to me, it seems that worn or bent shift forks.  Worn/loose shift drum, worn shift dogs, or shift drum pins.

Might be a short shifting foot issue.  Or, someone experimented with no clutch shifting.

Can't believe it's a synthetic oil issue.

Cheers,

Thanks TwoTired. Which of the above can be inspected (and replaced) without pulling the engine out? Shift drum and / or shift drum pins? Or just the pins?

And if none, how do I access the gears, from the bottom of the case or do I need to go in from the top and take the whole thing apart?

Thanks again,
Jonathan

[Terry in Australia]

Just don't use first! Just kidding, more than likely worn dogs, you can't properly inspect your gearbox until you pull the engine out, so either rip it out, flip it upside down and split the cases without bothering with the head and cylinder block, and pull your gearbox apart and inspect everything properly, or take it easy in first and quickly shift up to second.

[Jonm99]

Upon further testing today, I noticed that first gear is not just popping out of gear into some phantom neutral (which I would of expected), but it's actually shifting into neutral. So... I put my foot down on the shifter, holding it in first, and let it pop to see what happens. Transmission still pops out of first but as my foot is on the shifter, immediately pops back into first. Is this an indication of anything?

[Terry in Australia]

Yep, sounds like worn dogs mate, it's trying to jump out of gear, and you're forcing it back into gear. Time to come out, methinks. Gearboxes are innocuous in that you think they won't try to kill you, no matter how worn out they might be, but I got a nasty scare on New years Eve 2019. I'd just gotten off the freeway (70 MPH) and had slowed down to around 30 as I went thru a tiny town, when my gearbox locked up completely. It left a skid mark 30 feet long.

New Years Eve Bitsa disaster 2019 by Terry Prendergast, on Flickr

No warning, and pulling the clutch didn't do diddly. I never pulled the engine apart, I just swapped another engine into it. (which turned into a nightmare, the "Owner rebuilt" engine had been butchered, and I had it in and out 3 times before I finished fixing all the previous "Builders" bodges) There is a theory that Honda, or someone else, installed some of the circlips that hold the gears onto their shafts backwards, causing them to pop out to float on their shafts, and for two gears to engage simultaneously.

As I said, I never pulled mine apart, but the point I suppose, is not to assume that something as innocent as your bike popping out of gear isn't a problem. That skidmark on the road was nothing when compared to the skidmark in my undies.....

[PeWe]

It was a similar case not long ago where the problem was in the shifting mechanism not fully move the gear drum/keep it in place.

No problem using synthetic oil. Semi or full synth (esther)
It's just a better and more expensive oil.  Maybe not needed in an old stock bike.

Important it is labeled for bikes with wet clutch and correct viscosity.
Car oils have antifriction stuff that can cause clutch slip.

[Jonm99]

@Terry - Yikes! Thanks for the heads up... I'll be sure to ride with a diaper until the issue gets resolved 🙂

@PeWe - "It was a similar case not long ago where the problem was in the shifting mechanism not fully move the gear drum/keep it in place." - Can you please expand on this? Can it be just with the one gear (first)? What exactly am I looking for (aside from delaying the inevitable case split)?

Thanks once again,
Jonathan

[scottly]

Jon, as far as things you can check without splitting the cases:
1. Check the shift mechanism under the cover (tip: when removing the cover, press on the shifter shaft with your thumb while pulling the cover off with your fingers so the shaft stays in place)
2. Make sure the neutral switch is properly seated in the lower case, as it locates the shift drum laterally.
3. Make sure the clutch hub has the proper concave washer installed, as this affects the lateral position of the trans shaft.
4. Worn dogs usually cause the trans to pop out of 2nd gear, not 1st.
5. http://forums.sohc4.net/index.php?topic=86429.0

[HondaMan]

Things you can do without removing the engine:
1. Check for loose shift drum cover, and its pins.
This will cause it to only patially shift into 1st gear, while 2nd usually goes in OK. I have seen mostly K4 and K6 shift drums doing this, caused by an insufficent 'punch notch' having been applied, so the screw came loose. Add some Loctite to it, and make a new punch mark if yours is like this.

[Jonm99]

Thanks guys. Removed the cover (see attached images). Everything seems to be tightly secured, as far as I can tell. 1st and 4th was hit or miss when trying to engage (mostly first), but a short roll of the rear wheel and the gear "fell into place" each time. So... unless there is something in these pics that doesn't look right to you guys, I'm gonna open her up 

Thanks again for your help!
Jon

[Medyo Bastos]

There was a Honda service bulletin for this issue requiring the drum stopper to be modified. The detent had to be welded shut and the OD had to be turned down 1 mm





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[HondaMan]

There was a Honda service bulletin for this issue requiring the drum stopper to be modified. The detent had to be welded shut and the OD had to be turned down 1 mm





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I think this was only on the early K0 engines, IIRC?