Topic: CB750 Clutch Issues

[RAFster122s]

As HondaMan points out, it is assembled wrong...steel is first plate that goes in...

[HondaMan]

I do have 7 friction plates but the clutch was originally assembled with a friction plate as the first plate (on the motor side) and that is how I reinstalled it…

When you get the stack complete, the top cover plate should fit [about] halfway into the baskets 'fingers' at the top edge. In the Old days (sic...) with Honda's own plates the top cover plate would be just outside the fingers of the basket, with the top edge of the top cork plate dead-even with the ends of the fingers. That was their precision in assembly, pretty impressive! Today we are getting generic clutch plates that can range between 0.134" and 0.142" thick brand new, so this old rule-of-thumb doesn't fit anymore. The length of the basket's fingers are based on the 0.141" new-cork-thickness spec back then.

One of the finer points to watch for when assembling the stack with modern plates is: the top cork plate's surface should not sit beyond the finger's ends. If it does then the top plate can slowly damage the ends of the basket's fingers. I have seen this several times lately (since about 2005). If it happens, installing one cokr plate from a 550  (0.121" thickness) is enough to resolve it, so I put that one on top.

The later 750 clutch baskets also have the top ends of those fingers notched about 1mm on each side. These are there to fit the special "slope cut" corks plates, whose cork blocks are a parallelogram (or rhombus) shape instead of rectangular. These types of plates are "slipper" plates, designed to slide just a bit when the clutch is dropped too fast, obstensibly to reduce shock to the primary chain. This was/is a Honda engineer's 'saftey blamket' that came from the lowered primary drive ratio (and slightly elevated HP rating) of the post-1974 versions of these engines, like the F0 and later ones.

The point of that last paragraph is: many of these kitbashed bikes today also get kitbashed engine parts. I am currently reminded of this in the K1 engine I am working on now that had no friction plates at all, and a stack of steel plates from a K6/F0 engine with the [in]famous dual-spring steel plate: when assembling the early clutch with this dual-steel-spring plate in the stack you must lose one cork plate (6 total) in order for the stack to fit. When that happens, you must use the thickest new plates like the ones from PartsNmore (0.140"-0.142" new) and avoid use of the steel plates that do not have the checkered faces on them (i.e., smooth steel plates), to prevent slip.

[flyingclutch]

Hopefully the posted picture of fiche is available below... but anyway, I have a question for some folks.

Elkpent and HondaMan gave me the brilliant solution of making sure I wasn't just repeating someone else's mistakes. Decided to dig into the parts fiche for the clutch on a 72 motor like the one I have.

Perhaps this is a stupid question, but my clutch pack does not seem to include parts 3, 5, and 11 as listed in the fiche. Are these necessary? All I have between the outer hub and the inner pressure plate are 7 brand new EBC friction disks and 6 steels. The seating depths of a completed clutch that HondaMan mentioned appeared to be what I had when I installed the clutch this last time (friction plate a few mm's from the end of the basket fingers.)

Just would like to know if the absence of the parts mentioned (3, 5, and 11) would cause some sort of critical failure or maybe be the reason my magical Honda Pressure Plate Cracking Machine 750 has been causing me headaches for the past few months.

[bryanj]

 Think you will find they are onlyfor very early clutches, chech engie numbers used up to, should also be on parts fiche.

[ekpent]

Just to verify for folks #7 is the part you have broke twice now correct ?

[Old Scrambler]

Basics..........engine # please!!

If the shafts and bearings are worn or bent, the clutch basket components will break  

From what your describing its obvious that someone else was 'in' your motor and has left you with more than a few surprises. 

[HondaMan]

You're on the right guess: the #3, #5 and #7 parts only appeared in K0 engines or a few recycled later ones (as in, when they failed QC checks before installation and got sent back to the shop for some repair, to arrive later at the production line - like mine did). My (early) K2 came with a K1 engine stamped as K2 and originally had the 2 wires (#11) with that thicker back steel plate. Those comprise 6-cork-plate clutches instead of the later 7-cork-plate versions.

Just in case:
Your top plate should be the aluminum hub's flange: don't add a steel plate as the last one on the top. I have seen that happen before, too, and the extra height binds the clutch springs, breaking the pressure plate.

[flyingclutch]

Folks,

After 50-some replies, hundreds of dollars I didn’t have, and many hours and tears, I managed to ride the bike today.

I installed a new/old pressure plate with new EBC plates and springs. When I was tightening down the lifter plate I used a socket with a microfiber wrapped around it for grip, and I turned those suckers in 97% of the way by hand. Truth be told I think my issue was that the inner pressure plate was not lined up with the rest of the clutch pack, as was suggested here… so apparently a broken inner pressure plate will not allow the plates to engage at all!

Threw it all together and noticed that when I went to made the adjustment on the lifter nut that there was immediately a very different feeling.

The clutch lever feels amazing. The effort to pull it is easier than on some modern bikes I’ve owned/ridden. The engagement is also great, it bites super hard and feels like a race bike.

Now I need to teach myself how to sync 4 carbs because it bogs under high load  and I get some black smoke tufts at high rpm so I’d reckon that means she’s on the rich side. Apart from that and a mysterious oil leak near the shifter, she’s about ready to roll.

Thank you guys all so much.

[Stev-o]

Fantastic!   

Now the bad news, down your carbs are running rich due to not being sync'd.  That would just balance the vacuum so it will run smoother.   

Lets start out by asking if you have rebuilt the carbs and used aftermarket carb kits?  When was the last time you did a full "3000 mi tune up"?

[flyingclutch]

Fantastic!   

Now the bad news, down your carbs are running rich due to not being sync'd.  That would just balance the vacuum so it will run smoother.   

Lets start out by asking if you have rebuilt the carbs and used aftermarket carb kits?  When was the last time you did a full "3000 mi tune up"?

Man, I was having a good day  haha

I changed the oil on it before rebuilding the carbs. Rebuit the carbs using some generic kit from 4into1 because I wasn’t sure that the bike would even run. The kit is using the stock jets and needle size, and I left it on the stock needle height. Replaced float needles and none of them were really sticky. I have noticed that I get some fuel leakage if I leave the fuel valve on, so could be that a float is binding somewhere.

I have not yet changed the spark plugs.

The previous owner was running the stock airbox with no filter in it, stock jets and needle, and a 4 into 1 exhaust.

[flyingclutch]

I should also note that the tank is pretty low on fuel and has a relatively high percentage of Seafoam that could be causing the smoke.

[HondaMan]

I should also note that the tank is pretty low on fuel and has a relatively high percentage of Seafoam that could be causing the smoke.

Good job, Mr. Goodwrench!
Think of how much new stuff you've learned now (that you probably never wanted to know?). 

The richness/bogging stuff:
start with the basics. If it doesn't have an air filter inside the airbox, it needs one. Without it, it will run just like it had 'pod' airfilters, which I can assure you is a road you do not wish to ride down, as it has no end.

Make sure the spark timing is right: if you installed a points plate from 4into1.com then you need to change the condensors BACK to the old ones on your old points plate, or it will: run rich, bog down, idle poorly, and foul plugs. Right now (for the last 3-4 years) we are getting bad condensors on all points plates but the ones from TEC Japan, and those are scarce, probably sitting in containers on ships off the West Coast about now.

[flyingclutch]

I should also note that the tank is pretty low on fuel and has a relatively high percentage of Seafoam that could be causing the smoke.

Good job, Mr. Goodwrench!
Think of how much new stuff you've learned now (that you probably never wanted to know?). 

The richness/bogging stuff:
start with the basics. If it doesn't have an air filter inside the airbox, it needs one. Without it, it will run just loike it had 'pod' airfilters, which I can assure you is a road yuo do not wish to ride down, as it has no end.

Make sure the spark timing is right: if you installed a points plate from 4into1.com then you need to change the condensors BACK to the old ones on your old points plate, or it will: run rich, bog down, idle poorly, and foul plugs. Right now (for the last 3-4 years) we are getting bad condensors on all points plates but the ones from TEC Japan, and those are scarce, probably sitting in containers on ships off the West Coast about now.

Thank you HondaMan.


I did install a K&N filter into the airbox.

It currently has the points plate which came in it. I have a Dynatek that I was planning on dropping in…

I will recheck the spark timing. I do think it ran well and then I had to readjust the points for some reason and afterwards it didn’t run that great. Actually I had installed the coils that came from the Dynatek kit so maybe that’s what’s causing it?

I had initially chalked it up to low fuel and unsynced carbs as well as an inexperienced wrench who had never worked on a 4 cyl carbed bike. But thankfully with the clutch issue taken care of I feel better.

[ekpent]

Did you save all of the old carb parts you took out and replaced with aftermarket ? If so you can check to see if its genuine Kehin bits looking for logo's etc..

[PeWe]

My old K6 carbs still run with their stock jets except for main jet a few sizes bigger.
Around 100.000 km.
So why replace good jets that most likely need a clean only?
I could not see any wear on the needles.

Carbs even have the original float valves. I have new Keyster set of valves as well as Cruzinimage complete set I only used the rubber.

I tested a size bigger pilot jets from 4-1 that was crap. The thinner inner part stuck in carbs and snapped inside. I had to remove carbs and drill the holes on 2 carbs removing the brass debris. Drilled the other 2 to ensure same sizes on all 4. Old stock went back.
Not even Cruzinimage kit was that bad. Their pilot jets fitment is OK.

[flyingclutch]

My old K6 carbs still run with their stock jets except for main jet a few sizes bigger.
Around 100.000 km.
So why replace good jets that most likely need a clean only?
I could not see any wear on the needles.

Carbs even have the original float valves. I have new Keyster set of valves as well as Cruzinimage complete set I only used the rubber.

I tested a size bigger pilot jets from 4-1 that was crap. The thinner inner part stuck in carbs and snapped inside. I had to remove carbs and drill the holes on 2 carbs removing the brass debris. Drilled the other 2 to ensure same sizes on all 4. Old stock went back.
Not even Cruzinimage kit was that bad. Their pilot jets fitment is OK.

I can say with a substantial certainty that the jets were Keihin and all, but the pilots were 100% destroyed by sludge. I’ve worked with enough pilot jets at this point to know that I didn’t have the means to clean them out.

[flyingclutch]

Well as it turns out the gas was just really low and it runs much much better now that it actually has fuel in it.

It could use a tune but it pulls pretty darn well in first gear. Today was my first time ever really riding a CB750.

Unfortunately it feels like second gear is skipping under hard acceleration. Feels like a loose chain, but the chain is adjusted. It is, however, 30 years old.

I simply cannot get a break! So it goes.

[Edit] It seems that the chain might be hitting the stopper for the center stand. Maybe it’s not as adjusted as I had thought…

[Edit to edit] It’s only happening in second gear, and I believe only during acceleration. Nasty sound from left side of bike.

[ekpent]

The good news would be that if you have a 4-1 pipe you usually have to make a stopper for the center stand on it so it does not rub the chain as the original pipes had one to eliminate that problem. The bad news is that if it is wanting to jump out of second when under a load then you could be looking at worn gear dogs or bent shifter forks which require splitting the lower case. Not a super rare problem with 750's depending on how bad they were abused.
  As for pilot jets I have not really been defeated yet on varnished up ones. Real carb dunk to soften and good heat will melt it out enough to get a fine wire through then more prolonged soaking with a final trip through a sonic bath. Easy peasy 

[flyingclutch]

The good news would be that if you have a 4-1 pipe you usually have to make a stopper for the center stand on it so it does not rub the chain as the original pipes had one to eliminate that problem. The bad news is that if it is wanting to jump out of second when under a load then you could be looking at worn gear dogs or bent shifter forks which require splitting the lower case. Not a super rare problem with 750's depending on how bad they were abused.
  As for pilot jets I have not really been defeated yet on varnished up ones. Real carb dunk to soften and good heat will melt it out enough to get a fine wire through then more prolonged soaking with a final trip through a sonic bath. Easy peasy 

I only ever have been defeated by a single pilot jet on a 110 out of my 15 or so projects. Decided I’d rather replace them than spend 24 hours soaking in whatever the local forum witch doctor suggested I should soak them in. It does however run well now for being untuned, as it was virtually out of gas entirely.

And yes, I also saw a forum about a pesky circlip. Looks like I’ll be splitting cases after all.

[flyingclutch]

Is it possible that the lifter plate is not fully seated on the pressure plate posts and is causing the clutch to slip in second under load? I have some doubt that it is a gear problem.

When applying power in second it does not remove itself from gear entirely, and it feels like on and off slippage... like it revs and catches hard and jerks forward. I'm not ruling out the possibility of a damaged shift fork or some other case split-type issue, but I certainly want it to remain as a last resort.

Plus second is my favorite gear  haha

[Old Scrambler]

A slipping clutch tends to occur in all gears, but especially in the higher gears. Your description is more likely to be the result of worn cogs and a bent fork in the tranny.

We still don't know if the chain-sprockets are in alignment, but I would be looking for a complete tranny component replacement, including bearings on the clutch-shaft and out-out shaft. If you go this route, suggest looking for a K8 shift drum.

[flyingclutch]

A slipping clutch tends to occur in all gears, but especially in the higher gears. Your description is more likely to be the result of worn cogs and a bent fork in the tranny.

We still don't know if the chain-sprockets are in alignment, but I would be looking for a complete tranny component replacement, including bearings on the clutch-shaft and out-out shaft. If you go this route, suggest looking for a K8 shift drum.

What’s the significance of the K8 drum?

And I guess so. I’m just afraid to split cases. I’ve been fortunate enough to have managed to avoid splitting cases on anything I’ve ever worked on, and transmissions generally scare the hell out of me.

I believe this is the “denial” stage of grief.

[PeWe]

CB750 is easy to take apart as long as you have found all M6 screws keeping the cases together. The bigger ones are obvious.
M6 in starter motor compartment and beside clutch cover the tricky ones easy to miss.

Important to check a manual or parts chart to identify where all screws sit.

[Old Scrambler]

when you drain the oil...........please report if metal bits are found..........easy to detect if you have a magnetic drain-plug. Then remove motor from chassis and remove all top fasteners. Tip the motor over and remove bottom fasteners and case-half. Yes, the cir-clip and dowels will be troublesome, but then you will have accomplished something  

[HondaMan]

Is it possible that the lifter plate is not fully seated on the pressure plate posts and is causing the clutch to slip in second under load? I have some doubt that it is a gear problem.

Umm...not likely, sorry to say.

When applying power in second it does not remove itself from gear entirely, and it feels like on and off slippage... like it revs and catches hard and jerks forward. I'm not ruling out the possibility of a damaged shift fork or some other case split-type issue, but I certainly want it to remain as a last resort.

This does sound like a bent shift fork and/or worn-off ("rounded") gear dogs on the C2 gear. That's the sad part. It is most common on the 750 made before the K3 version, when Honda increased the width of the "neutral bump" on the shifter cam.

That said: the last one of those that I had to repair (K1) had been ridden 28 years longer by simply skipping over 2nd gear! He would just rev it a little further in 1st gear (which is good, it cleans the sparkplugs a little more) and then double-shift it into 3rd. He commuted some 50 miles per day in Arkansas, year-round on that bike, until he reached some 60k miles and it was leaking oil, so he sent it to me for a full rebuild. I put in a shifter drum from a K4 engine, had the gear dogs recut on C2-C5 gears, and away it went! Still running today, almost 10 years later.

So...if you can live with the double-shift approach for a while, the engine wil not give grief beyond not having 2nd gear handy. I might suggest changing the front (counter) sprocket down to 17 tooth size during this exercise, which will lower 3rd gear enough to make the engine comfortable and more power-on-tap in 30 MPH traffic. When I am stuck with a season (usually due to where I work) of in-city traffic, this is what I do with mine. Then when I get to a season of hiway travel I go back up to 18T (or 19T sometimes, but that requires other engine mods).