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Author Topic: CB750 Clutch Issues  (Read 6974 times)

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Offline flyingclutch

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CB750 Clutch Issues
« on: September 15, 2021, 10:46:47 AM »
Howdy folks,

Long time reader first time poster. In fact, I'm not sure I'm even in the right thread, so please berate me in the direction of finding the correct one, if applicable.

Anyway, I have a 1978 CB750K which I bought for $500 a few months ago. The bike was last tagged in 1987. It's been rough getting it running since the previous owner threw some random parts into it (motor from a 1972, forks from a 70-71, and a few other surprises,) in addition to my starting law school.

The bike runs now and starts on the first kick (with a little throttle and choke, and then doesn't like the choke anymore) but when I click it into gear, the RPMs drop and it starts lugging if its at idle. I do think it has a very, very slight pull forward while in gear. The clutch has no effect on the engine speed or anything else for that matter. There is some resistance at the clutch lever, the cable is new, and I have adjusted it at the throwout arm using the manual instructions, and have even made some crazy adjustments just to see if the condition would change. The throwout arm adjustments have little to no effect.

Is it possible the plates are stuck in a partially engaged position? I have no clue what to do at this point.

Any advice would be appreciated.
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Offline CaptFatCat

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Re: CB750 Clutch Issues
« Reply #1 on: September 15, 2021, 11:41:57 AM »
Any adjustment room left on the lever adjuster?
Does it roll in gear with clutch in with motor off?

I've dealt with similar issues.
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Offline flyingclutch

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Re: CB750 Clutch Issues
« Reply #2 on: September 15, 2021, 12:36:22 PM »
There is plenty of room to adjust in both directions on the lever adjustment. It noticeably has some resistance in gear but will roll with some effort. The clutch being pulled in is apparently has no impact on its ability to roll while in gear.
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Offline Stev-o

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Re: CB750 Clutch Issues
« Reply #3 on: September 15, 2021, 01:02:39 PM »
Yes, the plates are most likely stuck together.  Have you changed the oil?

What will happen if you try to take off in 1st gear? 

Try to ride it by pushing, you may be able to "unstuck" the plates, otherwise you may be looking at a clutch rebuild.
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Offline CaptFatCat

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Re: CB750 Clutch Issues
« Reply #4 on: September 15, 2021, 01:12:53 PM »
I would just make sure you have the proper freeplay in the lever and ride it until it gets hot a few times.

I had a K2 with same issue, it sat for many years, clutch was stuck, got it hot, let her sleep all night with a wire holding clutch lever in. Next day clutch worked again perfectly after some adjustment.
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Offline ekpent

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Re: CB750 Clutch Issues
« Reply #5 on: September 15, 2021, 01:13:34 PM »
 Having a little bit of a hard time determining your exact problem. If the clutch plates were stuck you would not be able to put it into gear running without killing the engine and lurching forward.
  Are you saying that when you put it in gear engine running and let out the clutch the bike does not move ??
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Offline flyingclutch

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Re: CB750 Clutch Issues
« Reply #6 on: September 15, 2021, 03:53:15 PM »
Quote from: ekpent on September 15, 2021, 01:13:34 PM
Having a little bit of a hard time determining your exact problem. If the clutch plates were stuck you would not be able to put it into gear running without killing the engine and lurching forward.
  Are you saying that when you put it in gear engine running and let out the clutch the bike does not move ??

The bike does not move when I put it in gear. When I put it into first gear, the engine speed drops, but the bike will not move on its own volition. There is no difference I can notice as to whether the clutch lever is pulled in or is let out.

I have tried this in both 1st and 2nd gear to the same result, essentially.

The engine oil has been changed. There was maybe 8 ounces of oil in the bike when I got it, as it had been sitting for decades with a leak.

Thanks CaptFatCat, I saw that overnight-tied-clutch lever trick and was curious to try it. Of course, I'm skeptical it will work for my situation.

If I had to describe what I thought the issue might be, I'd say it almost seems like the clutch is engaged just to the point where there is some clutch contact, but not enough to engage the transmission and is resulting in heavy slippage whenever the bike is in gear, and the clutch lever has no effect on this. Of course, I have no idea what's going on and just hope it's not the main drive gear...
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Offline goodtryer

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Re: CB750 Clutch Issues
« Reply #7 on: September 15, 2021, 05:05:07 PM »
With the engine off, the transmission in neutral and without touching the clutch lever, does the bike roll freely?
With the engine off, the transmission in first gear and with the clutch lever pulled in to the handlebar, does the bike roll freely?
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Offline flyingclutch

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Re: CB750 Clutch Issues
« Reply #8 on: September 15, 2021, 05:13:32 PM »
With engine off, trans in N, and no touching of the clutch lever, the bike rolls freely.

With the engine off, trans in first, and the clutch pulled in, the bike rolls a few inches and stops.
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Offline CaptFatCat

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Re: CB750 Clutch Issues
« Reply #9 on: September 16, 2021, 05:54:11 AM »
Sounds like it's stuck to me. I have remedied this issue in the past by rocking the bike in gear back and fourth with the clutch pulled in and eventually broke it loose. The sleep with the lever wired in for a day or so worked also. There is some YouTube video on how to free a stuck clutch.
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Offline goodtryer

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Re: CB750 Clutch Issues
« Reply #10 on: September 16, 2021, 06:17:46 AM »
Quote from: CaptFatCat on September 16, 2021, 05:54:11 AM
Sounds like it's stuck to me. I have remedied this issue in the past by rocking the bike in gear back and fourth with the clutch pulled in and eventually broke it loose. The sleep with the lever wired in for a day or so worked also. There is some YouTube video on how to free a stuck clutch.

+1
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"Tolerance will reach such a level that intelligent people will be banned from thinking so as not to offend the imbeciles."
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Offline flyingclutch

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Re: CB750 Clutch Issues
« Reply #11 on: September 16, 2021, 07:04:18 PM »
Well folks, after trying the overnight zip tied clutch lever trick and rocking the bike forward and backward about 5 times, the bike now appears to roll freely in gear!  :-\ Looks like she's due to have the clutch cover pulled.

This bike was supposed to be a relief from school stress, not add to it! Of course, who am I kidding, it's a vintage bike. At least it's a Honda  :P
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Offline Spanner 1

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Re: CB750 Clutch Issues
« Reply #12 on: September 16, 2021, 08:55:53 PM »
There is nothing wrong with your clutch except plates were stuck/semi stuck due to laying around for years. Just ride the bike for 50 miles or so and all will be well. No work needed, enjoy your bike !
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Offline flyingclutch

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Re: CB750 Clutch Issues
« Reply #13 on: September 16, 2021, 09:31:36 PM »
Spanner, I appreciate the sentiment and well wishes, unfortunately my issue was not resolved.

I decided to pull the clutch cover off since I had some time today, but did not have a lock nut tool to remove the clutch pack. Turns out mine was sufficiently loose to be able to very easily tap free with a hammer and a flathead (I accept whatever denigration you may levy against me for that).

The nut was not on there super snugly, and the locking washer tab thing only had one tab bent down to lock the nut into place.

The clutch plates were sufficiently soaked in oil, albeit having some rust spots on them. They were NOT stuck together...

Minimum plate thickness for the SEVEN friction plates in the pack was 3.2mm. I believe discard spec is 3.1mm, but feel free to correct me.

The only anomaly I noticed throughout the inspection was that an item, which I believe to be the pressure plate, was cracked. I'll try to attach a picture, though I'm not entirely certain that's the cause of my issue, but obviously I'll have to go about finding a replacement...

EDIT: Oh and there were no noticeable or "feelable" grooves in the clutch basket fingers.
« Last Edit: September 16, 2021, 09:33:59 PM by flyingclutch »
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Offline flybackwards

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Re: CB750 Clutch Issues
« Reply #14 on: September 17, 2021, 02:14:04 PM »
Lifter plate maybe - four little arms and the springs sit on it? They are easy to crack when assembling - post the pic..
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Online Don R

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Re: CB750 Clutch Issues
« Reply #15 on: September 17, 2021, 04:44:55 PM »
 When you replace the cracked lifter plate, tighten it carefully and evenly. Wrenching one bolt at a time all the way down will break a new one too.
 These bikes can be a source of relaxation but you have to be in the right frame of mind. Remember to enjoy the experience as well as the ride.
 I try to think of the guy that installed that part 50 odd years ago and how he would be encouraging me to get it going.
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Offline HondaMan

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Re: CB750 Clutch Issues
« Reply #16 on: September 17, 2021, 08:03:08 PM »
The inner plate (which has the 4 long 'fingers' on it where the springs reside) can be broken by tightening the bolts unevenly, and it can be broken if it was not meshed to the splines of the clutch basket before those bolts were tightened. Both are common reasons for failures, and they act just like yours, with a clutch that cannot engage, but drags.

There should be only 1 lock-tab bent inward to hold the clutch hub nut. That's made that way on purpose.

There is a cupped washer behind the nut that is lock-tabbed down. The hollow side of this washer MUST face the engine, or the clutch will drag when hot, in most cases. This is a spring washer, and tightening the big nut down to it holds the clutch spline hub to the outer with highly sprung pressure. When installing this nut, put the bike in gear, put a broomstick thru the rear spokes and swingarm to hold the wheel still, and tighten until you feel it 'bottom', then back off to the first lock-tab position that can be bent in to hold the nut from unscrewing itself.

I'd recommend soaking the clutch plates in oil overnight. That will restore them to whatever cork thickness they still have and will prevent premature plate wear from being dry so long.
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Offline flyingclutch

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Re: CB750 Clutch Issues
« Reply #17 on: September 18, 2021, 10:38:44 PM »
Hondaman, thank you, you are a legend in more ways than one. The washer and installation tip will surely save me some frustration. I'm also very much enjoying the blade fuse box I ordered from you.

It seems the part in question might be the infamous cracking lifter plate? I ordered the part from a microfiche and it's on its way.

Don, I have often thought the same thing when dealing with frustrating issues. I love doing this stuff, but, as I think is the case for most of us, the best part is the completed result. The struggles are easier to appreciate in hindsight  ::)

I have attached a picture of the broken part here to this post... not entirely certain how this works...

« Last Edit: September 19, 2021, 01:04:27 PM by flyingclutch »
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Offline bryanj

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Re: CB750 Clutch Issues
« Reply #18 on: September 19, 2021, 02:37:25 AM »
That pic isnt the lifter plate
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Offline ekpent

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Re: CB750 Clutch Issues
« Reply #19 on: September 19, 2021, 05:38:05 AM »
 The lifter plate that gets broken a lot goes on the outside/front of the clutch. Haven't seen people posting here  with your part being broken. Its technically listed as "plate,clutch" but I'll call it a clutch plate  :D
« Last Edit: September 19, 2021, 02:46:55 PM by ekpent »
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Offline flyingclutch

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Re: CB750 Clutch Issues
« Reply #20 on: September 19, 2021, 01:01:28 PM »
Quote from: ekpent on September 19, 2021, 05:38:05 AM
The lifter plate that gets broken a lot goes on the outside/front of the clutch. Haven't seen people posting here  with that part being broken. Its technically listed as "plate,clutch" but I'll call it a clutch plate  :D

I'm not entirely certain what its function is, I thought I saw on a parts list that it was some sort of pressure plate. Which would make sense because it would seem that the plates lie in there...
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Offline flybackwards

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Re: CB750 Clutch Issues
« Reply #21 on: September 19, 2021, 07:32:25 PM »
That looks like the back of the pressure plate - 22350-300-030 I think. Not sure how it gets broken like that. Check to make sure you have the correct number and thickness of clutch plates and the correct springs. That plate has clearly been overloaded somehow. Your clutch will work much better with a good one!
« Last Edit: September 19, 2021, 07:37:08 PM by flybackwards »
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Offline flyingclutch

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Re: CB750 Clutch Issues
« Reply #22 on: September 19, 2021, 07:34:37 PM »
Flybackwards, that is the part number I have ordered to replace this one.

And I hope so, as right now, it doesn't work much at all!
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Offline flybackwards

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Re: CB750 Clutch Issues
« Reply #23 on: September 19, 2021, 07:38:57 PM »
Yes - it will flex and do all kinds of weird things when it is asked to work...
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Offline flyingclutch

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Re: CB750 Clutch Issues
« Reply #24 on: September 19, 2021, 07:43:41 PM »
Quote from: flybackwards on September 19, 2021, 07:38:57 PM
Yes - it will flex and do all kinds of weird things when it is asked to work...

My fingers are about as crossed as I can possibly cross them... Though I suppose it would make sense that this is a substantial cause of my problem. It seems this is a relatively rare problem!

I guess that means I'm really lucky!  ;D
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