Author Topic: 2nd gear issue  (Read 7130 times)

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Offline 750IV

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Re: 2nd gear issue
« Reply #25 on: March 12, 2021, 07:54:15 AM »
It depends on the power engine has.
You'll feel the difference,  smooth slip or jerky movement by gearbox.

Drive chain OK, not too worn sprockets looking like a stormy sea and chain jump over the cogs...?

When my K6 had car oil of good brand in the 80's (Mobil 1)
It slipped when gearing to 5th at around 190kmh +WOT. I heard how it slipped, reduced throttle and wot again. It sounded like gearing to 6:th ;D.

Same bike 2 years ago, slip on 2nd gear around 3000-3500 rpm. Much more torque with 1000cc.
That was the clutch lever with too little play. After that no chancing with 1/4 turn out. Always 1/2.

That bike is well worth to get this sorted.

In worst case open the cases with engine up-side down to fix eventual gear forks.
Before that: I should investigate if something with gearing mechanism inside the shifter cover can be involved.
To answer all your helpful comments:
The engine has good power, very responsive throttle, easy cold start, nice idle.
Jerky in second only, all other gears strong, no clutch slippage.
Drive chain new and sprockets not worn at all. Curiously though I set up the new chain for 1/2" of slack but just checking now it's about 1". Should a new chain stretch that much initially and would that be enough to jump sprockets? Did the jerking in second cause the chain to stretch that much?
Non energy saving oil, no friction modifiers, 10W40. What is your preferred brand and weight?
I believe I did 1/4 turn but will recheck and do 1/2 turn on the clutch lever.
Shift mechanism looks fine, see above pics.
I understand the ultimate fix is splitting the cases but I just don't have room/garage space to do that.

Offline PeWe

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Re: 2nd gear issue
« Reply #26 on: March 12, 2021, 09:36:26 AM »
Motorcycle oil for wet clutch, 20W-50.
10W-40 should not slip.

20W-50 recommended when ambient temp is over 15C.
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline rotortiller

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Re: 2nd gear issue
« Reply #27 on: March 12, 2021, 09:58:04 AM »
Quote
Curiously though I set up the new chain for 1/2" of slack but just checking now it's about 1". Should a new chain stretch that much initially

Yes, as the grease squashes the chain will initially get loose then settle down. As your bike does not slip in first or third I'd be looking at the gearbox area.

Offline seanbarney41

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Re: 2nd gear issue
« Reply #28 on: March 12, 2021, 12:24:19 PM »
I have a K3 engine that had a similar sounding problem.  I never figured it out as the camshaft ate itself so I switched it out for another engine and have not investigated further.  I am pretty sure it was not a clutch problem.  Clutch slippage would not be this violent.  But can anyone explain how a gearbox can seem to pop out of second and slam back in repeatedly in rapid succession?  If i am not mistaken, this is what the op is dealing with.
If it works good, it looks good...

Offline 750IV

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Re: 2nd gear issue
« Reply #29 on: March 12, 2021, 01:06:03 PM »
I have a K3 engine that had a similar sounding problem.  I never figured it out as the camshaft ate itself so I switched it out for another engine and have not investigated further.  I am pretty sure it was not a clutch problem.  Clutch slippage would not be this violent.  But can anyone explain how a gearbox can seem to pop out of second and slam back in repeatedly in rapid succession?  If i am not mistaken, this is what the op is dealing with.
You're not mistaken that's exactly what I'm dealing with.
From what I'm hearing it's referred to as a false neutral where the engine revs as if in neutral then slams into second repeatedly until you get off the throttle, upshift or go back to first. First, third, fourth and fifth are fine.

Offline 70CB750

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Re: 2nd gear issue
« Reply #30 on: March 12, 2021, 01:30:35 PM »
I had this on my K0.  It started slow and got worse.  The second gear just would not stay in. all other gears and the clutch were ok.  I ended up riding it longer than I should - while building my K3F - going from 1 directly to 3 and up.  I believe the dogs were worn out.   The K0 is still boxed up, waiting to be rebuild, but I will be replacing gears with later ones.
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Offline seanbarney41

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Re: 2nd gear issue
« Reply #31 on: March 12, 2021, 04:31:39 PM »
I have a K3 engine that had a similar sounding problem.  I never figured it out as the camshaft ate itself so I switched it out for another engine and have not investigated further.  I am pretty sure it was not a clutch problem.  Clutch slippage would not be this violent.  But can anyone explain how a gearbox can seem to pop out of second and slam back in repeatedly in rapid succession?  If i am not mistaken, this is what the op is dealing with.
You're not mistaken that's exactly what I'm dealing with.
From what I'm hearing it's referred to as a false neutral where the engine revs as if in neutral then slams into second repeatedly until you get off the throttle, upshift or go back to first. First, third, fourth and fifth are fine.
I don't think this is really just a "false neutral".  False neutral can occur at any time on any cb750 between any 2 gears...if you don't shift it firmly and positively and it will just stay in the false neutral until you try the shift again successfully, not a repetitive and violent in and out and in and out of second.  So now there are 3 of us who have not found or fixed the problem and still lots of guys trying to tell you how to fix something else.
If it works good, it looks good...

Offline scottly

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Re: 2nd gear issue
« Reply #32 on: March 12, 2021, 09:07:53 PM »
If you hold the shift lever up with your toe after the 1-2 shift, does it still pop in and out of gear? If you downshift from 3 to 2, does it pop in and out?
(BTW, you are correct, it IS a type of Bellville type of spring washer. :-[ :-[)
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Offline Henning

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Re: 2nd gear issue
« Reply #33 on: March 13, 2021, 01:55:18 AM »
This does sound to me like a bent fork in the gearbox; had this problem many years ago. Changed out the selector fork and all was good for many more miles.
71 or thereabouts 750 K1 - this one should have been put down

Offline 750IV

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Re: 2nd gear issue
« Reply #34 on: March 13, 2021, 06:16:02 AM »
If you hold the shift lever up with your toe after the 1-2 shift, does it still pop in and out of gear? If you downshift from 3 to 2, does it pop in and out?
(BTW, you are correct, it IS a type of Bellville type of spring washer. :-[ :-[)
I'll try those things on the next road test.
Right now a few things are apart and I drained the oil to check for any metal shavings which I didn't find.

Offline 750IV

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Re: 2nd gear issue
« Reply #35 on: March 13, 2021, 06:19:57 AM »
This does sound to me like a bent fork in the gearbox; had this problem many years ago. Changed out the selector fork and all was good for many more miles.
If it turns out to be that the next owner can get into it.

Offline PeWe

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Re: 2nd gear issue
« Reply #36 on: March 13, 2021, 06:32:36 AM »
Price will be much lower with that  issue.
If you fix it, only oil seal kit, Hondabond and new fork. Plus oil filter with oil.

You can get all forks with gear selector for not much on eBay.

Then sell bike as a good runner.
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline 750IV

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Re: 2nd gear issue
« Reply #37 on: March 13, 2021, 06:51:04 AM »
Price will be much lower with that  issue.
If you fix it, only oil seal kit, Hondabond and new fork. Plus oil filter with oil.

You can get all forks with gear selector for not much on eBay.

Then sell bike as a good runner.
I'll probably sell it for what similar condition ones are going for nowadays less what a shop would charge for the repair?
The cost of the parts isn't a concern it's just that I don't have the room or time to do it.
Someone can ride it as is for the season then do the repairs over the winter.

Offline Remcod

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Re: 2nd gear issue
« Reply #38 on: March 14, 2021, 08:22:43 AM »
I have been messing with this for many years. Replaced the clutch over and over untill i noticed its in 2nd only. I have set it aside for now, untill i follow hondamans recommendations on the worn dogs/forks. The engine out part is stopping me now, need to buildup energy for that, but there is no escape here.

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Offline HondaMan

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Re: 2nd gear issue
« Reply #39 on: March 15, 2021, 04:48:25 PM »
I had this on my K0.  It started slow and got worse.  The second gear just would not stay in. all other gears and the clutch were ok.  I ended up riding it longer than I should - while building my K3F - going from 1 directly to 3 and up.  I believe the dogs were worn out.   The K0 is still boxed up, waiting to be rebuild, but I will be replacing gears with later ones.
You can also send them to FastByGast in Buffalo. Paul does an excellent job of fixing them up, I have had several done by him. You send the countershaft with the C2 and C5 gears and he squares off the dogs and slots, in perfect 120 degree fixturing.
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

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Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
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Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

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Offline 750IV

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Re: 2nd gear issue
« Reply #40 on: March 15, 2021, 07:35:39 PM »
I had this on my K0.  It started slow and got worse.  The second gear just would not stay in. all other gears and the clutch were ok.  I ended up riding it longer than I should - while building my K3F - going from 1 directly to 3 and up.  I believe the dogs were worn out.   The K0 is still boxed up, waiting to be rebuild, but I will be replacing gears with later ones.
You can also send them to FastByGast in Buffalo. Paul does an excellent job of fixing them up, I have had several done by him. You send the countershaft with the C2 and C5 gears and he squares off the dogs and slots, in perfect 120 degree fixturing.
Thanks for that info Hondoman.
Does he have a web page? 
I hope it's not the place in Grand Island, NY I found on google because a customer reported he's had his gears for 11 weeks and has been told it could be 6 more weeks.

Offline 750IV

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Re: 2nd gear issue
« Reply #41 on: March 16, 2021, 06:30:42 AM »

You can also send them to FastByGast in Buffalo. Paul does an excellent job of fixing them up, I have had several done by him. You send the countershaft with the C2 and C5 gears and he squares off the dogs and slots, in perfect 120 degree fixturing.
[/quote]
Does anyone  happen to have close up pics of the C2 and C5 gears showing the damaged dogs and slots?

Offline ekpent

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Re: 2nd gear issue
« Reply #42 on: March 16, 2021, 07:03:21 AM »
  Some good build pictures here from a guy who had second gear issues. Will show what it looks like inside the lower case well. http://forums.sohc4.net/index.php?topic=54689.25

Offline 750IV

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Re: 2nd gear issue
« Reply #43 on: March 16, 2021, 08:48:33 AM »
  Some good build pictures here from a guy who had second gear issues. Will show what it looks like inside the lower case well. http://forums.sohc4.net/index.php?topic=54689.25
Thanks for the link.
I can't really tell but in his gear pics I'm wondering if he's showing both 2nd and 5th gear being bad. It looks to me like the slots are ok in 2nd gear?
Same goes with 5th. It almost looks like the pins are ok.
« Last Edit: March 16, 2021, 10:46:40 AM by 750IV »

Offline 750IV

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Re: 2nd gear issue
« Reply #44 on: March 17, 2021, 04:27:45 PM »
If you hold the shift lever up with your toe after the 1-2 shift, does it still pop in and out of gear? If you downshift from 3 to 2, does it pop in and out?
(BTW, you are correct, it IS a type of Bellville type of spring washer. :-[ :-[)
I had a chance for a short test ride today after changing the oil, adjusting the clutch and chain and the answer is yes to both of your questions. If I keep it under 3k rpm in 2nd it's ok but above that it starts the banging in and out. I've decided to take it apart so in a few days I'll have some questions.

Offline HondaMan

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Re: 2nd gear issue
« Reply #45 on: March 17, 2021, 06:44:27 PM »
  Some good build pictures here from a guy who had second gear issues. Will show what it looks like inside the lower case well. http://forums.sohc4.net/index.php?topic=54689.25
Thanks for the link.
I can't really tell but in his gear pics I'm wondering if he's showing both 2nd and 5th gear being bad. It looks to me like the slots are ok in 2nd gear?
Same goes with 5th. It almost looks like the pins are ok.

These are classic pictures of the problem. The face of the C5 gear is much-rubbed by the pegs on the C2 gear in both of those pictures.
What then causes the troubles is: the pegs (dogs) are rounded and/or tapered with the wear, and they in turn taper the edges of the slots in the C5 gear. This makes the dogs and slots wedge each other apart under power. If the L fork is also bent (which is often the case) then the combo will not hold torque.

I do know of a couple of 750s that were driven anyway like that: they just rapped 1st gear and 2-shifted into 3rd, for years and years. The bikes were both city commuting bikes, and had well over 50K miles before the owners decided to fix the gearboxes.
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline 750IV

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Re: 2nd gear issue
« Reply #46 on: March 17, 2021, 07:46:52 PM »

[/quote]

These are classic pictures of the problem. The face of the C5 gear is much-rubbed by the pegs on the C2 gear in both of those pictures.
What then causes the troubles is: the pegs (dogs) are rounded and/or tapered with the wear, and they in turn taper the edges of the slots in the C5 gear. This makes the dogs and slots wedge each other apart under power. If the L fork is also bent (which is often the case) then the combo will not hold torque.

I do know of a couple of 750s that were driven anyway like that: they just rapped 1st gear and 2-shifted into 3rd, for years and years. The bikes were both city commuting bikes, and had well over 50K miles before the owners decided to fix the gearboxes.
[/quote]

Thanks for the explanation. I'll know what to look for when I get it apart.

Offline scottly

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Re: 2nd gear issue
« Reply #47 on: March 17, 2021, 09:17:02 PM »
Your issue doesn't sound like worn dogs to me; worn dogs pop out of gear under high loads, like WFO with an 836 at 7000 RPM, allowing the motor to hit 10,000+ RPM before you can react, not under light loads. Your symptoms sound like the trans isn't fully engaging the dogs?? Possibly a bent shift fork, but I have not had a problem with one, or improperly installed snap rings on the trans shafts, which I have had a painful experience with. :o
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Offline ekpent

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Re: 2nd gear issue
« Reply #48 on: March 18, 2021, 05:48:55 AM »
 On my 750 that was a problem child for 2nd gear it would gently go to the false neutral at a pretty low rpm under load. No clanking or other disturbing noises though. The engine is on a pallet and has not been opened so no forensic data to share. One of those someday projects as I swapped an 836 into that bike  ;)

Offline 750IV

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Re: 2nd gear issue
« Reply #49 on: March 19, 2021, 02:53:21 PM »
Your issue doesn't sound like worn dogs to me; worn dogs pop out of gear under high loads, like WFO with an 836 at 7000 RPM, allowing the motor to hit 10,000+ RPM before you can react, not under light loads. Your symptoms sound like the trans isn't fully engaging the dogs?? Possibly a bent shift fork, but I have not had a problem with one, or improperly installed snap rings on the trans shafts, which I have had a painful experience with. :o
I'll know more what's going on in the trans this weekend.