Topic: 1975 Cb750 k chain break - now shifts badly

[HondaMan]

well I put back everything and decided to ride it 10 miles. It is running great here is a video link - hope it works:  https://vimeo.com/manage/videos/1100882325  however I have to drive it in 2nd gear. I only putzed  around 10 miles in my neighborhood. It did not really want to go into 1st - mostly started in 2nd  - could eventually get into 3rd and 4th  not 5th -  would go back into neutral and neutral light works -false neutral in between 2nd and 3rd -

The false-neutral situation, along with the balky 1st-gear selection, suggests a bent "L" shifter fork. You'll only be able to determine that, though, by pulling the engine, removing the transmission, clutch, and alternator side covers, the shifter arm, then those [same] selector indexing parts, the 4 screws that hold the countershaft bearing in the lower case (and pull that out) and then separate the cases. You can then shift the gears in the lower case by turning the shift drum: watch the 2 gears nearest where you removed that countershaft bearing. If it shifts the most-outside gear (C5) deeply into the next gear toward the engine's center (C2) for 1st gear, but barely engages the C5 gear for 2nd, that is the typical bent-fork scenario. This happens when the bike falls over to the left side and the shifter hits something solid while the engine is not turning: the weight of the bike on the non-engaging C5 gear bends the tines of the "L" shift fork. There might also be evidence such as scrapes on the grooves of the C5 gear or on the sides of the "L" shift fork. (The "L" is cast on the side of the fork, but not visible when the fork is mounted to its slider shaft: the gears are in the way). Sometimes (pretty rarely) the sides of the groove on the C5 gear are burred up or burnt-looking, too, helping you to identify it.

Most of these that I have repaired got bent when the bike fell over in the owner's garage and the shifter hit something solid, like a brick or paint can, stored next to the bike. It tried to shift into 2nd with the weight of the bike on the shifter! That's a LOT of force.

[cado007]

does the oil pan need to come off?
Also this is a 1975 cb750k - does a replacement shift fork , drum or gears have to be from a 1975 cb750 k or will other years work?
I did see this long thread where Ron cieri -unfortunately used a 750 f trans and had to redo it : http://forums.sohc4.net/index.php/topic,59158.0.html
I have done a lot of research and this will be intimidating - but I think I can do it
Lastly would it be wise to redo the clutch before trying this? I do think it is a bent fork or maybe something like Ron had which seemed to be chipped gears which Hondaman you said you have never seen before. I have taken a cb 350 all apart split the cases and unfortunately it still is apart  - waiting to vapor b;;last the cases one day. It is actually way down on the list of projects. Anyway When I tackle this I want to take it apart ( taking as little off as necessary) I have read Rons 13 page thread and other research. and have everything organized and get back together as soon asap . I will replace any seals I can when in there.
Thanks
Damon

[HondaMan]

does the oil pan need to come off?
Also this is a 1975 cb750k - does a replacement shift fork , drum or gears have to be from a 1975 cb750 k or will other years work?
I did see this long thread where Ron cieri -unfortunately used a 750 f trans and had to redo it : http://forums.sohc4.net/index.php/topic,59158.0.html
I have done a lot of research and this will be intimidating - but I think I can do it
Lastly would it be wise to redo the clutch before trying this? I do think it is a bent fork or maybe something like Ron had which seemed to be chipped gears which Hondaman you said you have never seen before. I have taken a cb 350 all apart split the cases and unfortunately it still is apart  - waiting to vapor b;;last the cases one day. It is actually way down on the list of projects. Anyway When I tackle this I want to take it apart ( taking as little off as necessary) I have read Rons 13 page thread and other research. and have everything organized and get back together as soon asap . I will replace any seals I can when in there.
Thanks
Damon

The oil pan can stay on: it will also protect the bottom of the engine while you're pulling it out of the frame.
You can use a shift fork from any earlier CB750, but the ones from the K4/5/6 and the F0/1 are the better-made ones. The early ones had shorter dowels that follow the shifter drum grooves: Honda made them longer in the K4 and also widened the Neutral notch in the shifter drum to make Neutral easier to find. For that reason, the later (K4 onward) shifter drums also have deeper grooves, which tend to hold a bit more oil and make for smoother shifting overall.

This particular job takes some serious commitment to accomplish: you'll be deep inside the beast.

There is a book on how to do this.

[cado007]

Thanks Hondaman-I have your book and the clymer manual and the online manuals from here
Thanks
Damon

[bryanj]

Forget the clymer, if dry it is useful for lighting the bbq

[cado007]

thanks BryanJ- thats good to know

[HondaMan]

Forget the clymer, if dry it is useful for lighting the bbq

Hey, maybe I can finally use mine for something! It hasn't been off the shelf in 15+ years.

[bryanj]

Well i believe Haynes bought out Clymer annd they are worse!
That said i have all the different 500 ones except Marks, maybe a christmas present to me

[scottly]

I did adjust clutch, not sure if it could be clutch plates but I did adjust the clutch per manual and it feels good.

Does the bike shift through all 5 gears when the engine isn't running? Can you put the bike in gear, pull the clutch in, and push the bike easily? If the clutch adjustment is too loose, the clutch will drag and make shifting difficult. If the adjustment is too tight, the clutch will slip under load. The lever at the clutch cover should only have 1/16" or so of free-play when pushed with your finger: this adjustment is done first, and then the cable is adjusted at the ends.

Remember, this problem started after the engine had chain damage welded, without being taken apart. It's hard to see how that could cause a bent fork? If the inside of the case was hitting a gear, then pulling the pan, which you can do with the engine in the frame, would show aluminum glitter from the gear grinding into the case. Just trying to cover all the bases before you dive in headfirst.

[cado007]

Hi Scotty,
I did adjust the clutch. The bike does roll while in gear with clutch pulled in. It makes a grinding sound and won't go into 1st - goes into 2nd 3 and 4,
I had one of the guys I ride with ride it and they thought they heard noise in the clutch basket. I thought maybe I should just check and rebuild the clutch if necessary before deciding to pull the engine. also this bike will not start without pulling clutch in - 1975 cb750k - is that a normal feature? Thanks for the reply!
Damon

[bryanj]

Yes its normal, either the neutral light on OR clutch pulled in to start

[cado007]

So I am Planning on removing the engine this weekend- I feel comfortable with this.
Again since it has been a while 1975 cb 750 k- splitting cases to sort out bad shifting- question when splitting bottom case - I am trying to just remove only what is necessary - fix problem reassemble - I removed clutch plates and plates look great  -bike supposedly has 22000 miles so not original I am guessing and springs measured 35 mm  and fiber plates were 5mm which does not seem spec for 1975 cb750 k so maybe it has wrong ones? but it did shift into 2 3 4 . Well anyway for splitting cases do I have to remove rotor?  do I have to remove starter and oil pressure switch  and can I leave oil pan on - I have already removed and cleaned screen. can oil pump stay in?
 I looked up some good threads but the photos are all gone and I don't want to botch this
Thanks in advance!
Damon

[HondaMan]

To get to the trans:
1. Remove the several 6mm bolts (10mm socket size) on the upper crankcase, The K5 has 9, I think. Remove the 3 bolts across the back of the engine.
2. Turn engine upside-down.
3. Remove side covers (transmission, clutch, alternator, chain cover, that big nut over the points, and the points plate (punch mark it's position first so you don't have do a full realignment at reassembly).

Now pull the cases apart. Honda's goo is still in there, holding them together: I gently pry between the drive sprocket and the lower or upper case with a LONG screwdriver. If you forgot even one 6mm bolt somewhere, it won't budge, so make sure I didn't miss telling you one somewhere?

[cado007]

Thanks Hondaman,
Are there bolts under the starter ? do I have to remove the starter and oil pressure switch - guessing not since you did not mention.
Also is it better to remove hose from oil tank at the tank or at the engine? Also my clutch springs were 35mm and fiber plates were 5mm is that in spec for 1975 cb 750 k?  Lastly is Hondabond 4 the correct sealer to reassemble? sorry for the newbie questions and Thanks! Damon

[HondaMan]

Are there bolts under the starter ?

Nope!

do I have to remove the starter and oil pressure switch - guessing not since you did not mention.

You've got that right, too.

Also is it better to remove hose from oil tank at the tank or at the engine?

At the engine, always. Removing them at the tank causes a host of troubles.
You'll have to remove the oil tank from the frame: remove the tool tray above it to help with space. Then it lifts up a little and turns so you don't bend the oil lines a whole lot: they are old now, and expensive to replace if they get kinked.

Also my clutch springs were 35mm and fiber plates were 5mm is that in spec for 1975 cb 750 k? 

The 35mm springs sound like Vesrah's HD type springs. How many coils do they have?
The 0.5mm thick plates belong to modern superbikes, circa late 1990s onward, with 7-10 clutch plates in them. There is someone out there who is selling "Heavy Duty" clutches for these bikes by installing thinner plates and more of them: some (like Vesrah) are trying to tell the rider to install their thinner plates with their HD springs and call it 'good' - but it is not. How many fiber plates are in your clutch? There should be 7. There should also be a double-steel plate at the 2nd or 4th position in the plate stack: it will have tiny rivets on it that hold the tinier springs in between those plates. If this is gone and there are just 7 cork plates of 0.5mm thickness, despite even HD springs, the clutch will slip under heavy throttle.

Lastly is Hondabond 4 the correct sealer to reassemble?

That's the good stuff!

[cado007]

Thanks so much Hondaman! You were right 7 fiber disks and no double metal ones( can I buy the correct ones or a suitable replacement?)- I posted pics .
I made some progress - now I have to wait for someone to help me flip motor over as I am worried I would drop it .
I removed the engine tho. Should have it opened up in a couple of days and post pics back as I am not sure I will know what I am looking for( maybe some slag from the weld in grar or bent shift fork) I also posted pic of weld - it looks like it does not interfere with the seam. Plugging away Damon

[scottly]

A few years ago a member had a problem were the trans wouldn't go into 5th gear. It finally turned out to be a slightly domed washer on the clutch center under the lock nut that was installed backwards: the domed side is supposed to face out, but was facing in. He only had a problem with 5th, IIRC, but I would check it out before you split the cases. 

[cado007]

Well I guess it was good that we split the cases. My friend Flip had to use a pneumatic sander to sand  the weld in order split  the cases. I would say the shift drum is toast - shattered actually. I guess I was going try to replace the drumfirst as the forks do not appear to be bent and gears appear ok, unless it is better to replace forks too I was going to replace all the seals and the big dowel o ring - any other o rings or seals to replace? and clean mating surfaces . Thanks. Damon

[scottly]

Wow, that is odd!!?? I don't recall ever hearing of a broken shift drum before, and I don't see how a thrown chain or the repair could have caused it? What does the other half look like? Is there any evidence of gears hitting the case? The fork that engages the drum where the damage is fits to a gear on the other trans shaft, does all look well there?
Also, have a read through this thread about the snap rings:

http://forums.sohc4.net/index.php?topic=86429.0
   

[bryanj]

I have never seen any shift drum on any Honda do that unless the engine litterally grenaded

[cado007]

Yes this is how the shift drum  was when I removed. Luckily I had my friend Flip ,who is a lot more experienced help me.  The weld on the case was an issue but he fixed is an it looked like new - installed replacement shift drum - cleaned surface meticulously and new seals and orings - hondabond on bottom - then torqued big bolts 12 then 18 ft lbs  in pattern in manual- smaller bolt 8 ft pounds. Tested transmission it shifted thru all gears. I am suprised it shifted at all with it like it was. Fingers crossed it does not leak  - I am learning a lot - it was very educational - but i did have help. Thanks to everyone on the forum I guess now we have seen a broken one.
Damon

[HondaMan]

Wow!
I'm still trying to think through what series of events could happen to shear the shift drum without breaking/bending the forks?

Or, was it a marginal casting that didn't show external evidence of the weak interior?

Your friend must be a master welder, too!

[bryanj]

Is it possible that the break jamed the box and the resulting lock uptwisted gears/selector/ drum(weakened) then it fully sheared later.

This is just guessing of course

[cado007]

I am putting new sprockets and chain. I have a 100 link 530 tsubaki - it had an 18t front and 48t rear- is that the stock setup for 1975 cb750k ?
Also it has that chain oiler on it which I have never dealt with can i just screw oiler screw all the way in ? anyone ever use that joker oiler delete? you have to tap it in with permatex - not sure how easy it is to remove? or any other oiler delete ideas?
Thanks
Damon

[rotortiller]

Could simply be a casting flaw or bad metallurgy, cracks happen sometimes without excessive stress applied. Sharp edges can introduce such an incident.