Topic: '78 CB750K Grinding noises from transmision and jumps gears

[RezaM]

Hi all,
This is a '78 CB750 that I found laying in the bushes for 25 years, so i have had to disassemble, clean/repair/replace everything. It had a cracked case. full of water. carbs full of mud, etc. Just to give a perspective that there is no known knowledge of previous running condition.

So I rebuilt the bike, got it running really nice after a few trials of re-jetting carbs to run pods and for 5000ft altitude, etc. It ran very well as far far as the motor is concerned, shifted through all gears smoothly for a few test runs. It always had a little growl coming from the clutch/transmission area when I first started it, but it went away when the bike got warm, 5-10min.

Then as I ran the bike, I developed major oil leaks. The cause of that was an incorrectly built oil pump pressure valve that has since been found, fixed and cured the leak problems. That history is under another thread at the forum.

One thing to note is that the only anomaly beside growling noise mentioned above, the neutral safety switch would not fit all the way in. I discovered that the shoulder of the switch, next to the ball, was hitting the flange of the shift drum, so I ground off a few millimetres on one side, put it back in and solved the neutral light being on all the time problem.

I also have to say that the only part I did not disassemble was the clutch basket. When I took out, it looked very clean and I thought that if I needed to, I could do it after the bike was assembled.

I don't know if I am imagining, but the transmission problem has gotten worse since. Now the clutch growl is loud and has gotten worse. It also seems to want jump gears. I took it for a slow ride yesterday and it jumped from 2nd to 1st on its own, locked the rear wheel, skid for a while and killed the bike.

When it's making the grinding noises, I pull in the clutch and put it in any gear (usually 1st), and it becomes immediately quite while it's in gear and clutch. As soon as I let the clutch go, the noise returns, the chain runs but has a skipping jerky feel to it. then when I pull the clutch lever again, the noise doesn't go away.

Can't quite wrap my mind around what is happening in there. The fact that the neutral switch doesn't fit right tells me that somehow the shift drum is sitting a tiny bit too far out towards the outside of the bike, but not sure if that has anything to do with it.

I have adjusted the clutch a million times: Center screw all the way in until it barely gets a resistance, then 1/2 turn backed out. I have even tried messing with the adjustment while the bike is running in neutral to see if I hear any difference in noise, but nothing.

[newday777]

Rather than the clutch I suspect it is an internal tranny issue from you wording of the neutral switch and shift drum.
Possible missing parts???
Or improperly reassembled???
Did you take pictures of before and after of the transmission assembly?

[754]

Bearings that had corrosion  , are probably rapidly dying.. increasing clearance , letting gears mesh wrong.
 And all the metal bits are going thru your motor..
 Best pull it apart , or don't run it.

[Alan F.]

That was my inclination too Frank.
Do you think it'd be worth taking the chain off and putting it in neutral then turning the output shaft by hand to feel for crunch?

RezaM, did you replace or reuse the transmission bearings?

[754]

It's got to come apart, a used  tranny with bearings is likely cheaper than buying new bearings, I had motors like these , even the mains had  a wrinkly surface texture, and I expect they would  deteriorate  very rapidly ..
 What did you do about cracked case . WAS IT chain damage  ?

[scottly]

One thing to note is that the only anomaly beside growling noise mentioned above, the neutral safety switch would not fit all the way in. I discovered that the shoulder of the switch, next to the ball, was hitting the flange of the shift drum, so I ground off a few millimetres on one side, put it back in and solved the neutral light being on all the time problem.

This is your second self-inflicted wound!! The neutral switch is what locates the shift drum laterally in the case. By grinding a few millimeters off, the drum can't be held in the proper location.
Regarding the clutch, there is a cupped washer on the shaft where the basket sits; is that washer present, and facing the correct way? Is the clutch cover the original '78 one? Early clutch covers will rub when used with a late clutch basket.
Were the snap rings removed from the transmission shafts? If installed incorrectly, the rings can pop out of their grooves.
http://forums.sohc4.net/index.php?topic=86429.0 

[flybackwards]

Agree on the shift drum - having spent some time in the last week removing and replacing the stuck and corroded neutral switch on my K7 you do have to make sure the switch / drum retainer goes in all the way to properly locate the drum and shift forks in relation to the transmission shafts - or you get the symptoms of jumping gears and bad shifts as if the shift forks were worn. The switch has to go all the way in and when the switch is correctly positioned the drum actually looks like its a bit proud of the outside edge of the casing. I had the advantage that the motor is stripped and the cases are apart to see what was going on.

[RezaM]

Well, to answer a few question:
1) The lower case was cracked right at the drive gear area. According to the previous owner, it broke a chain at high speed. Got a very clean case from e-bay, and since the bearing colors happen to match the old case, I used the lower half of that one with the upper half of the original. This to keep the engine S/N matched to the frame VIN.

2)I did not replace the trans bearings. I checked them out, they were in very good shape. No rust/pitting/defects. They rolled very freely with no clicking/hesitating/jumping, etc. I oiled them and put the whole trans back in. Like I mentioned, I didn't rebuild the clutch either.

3) when I reassembled the motor, everything in the lower end went together very nicely. None of the gears had any big signs of wear or chipping, etc. Didn't have to force anything to go in place.

I am of the mind that there is something going on with the shift drum/neutral switch as well. I have attached a pic of the Before/After N.S. work. Here's why I think this is the case:

When I fired the bike up for the first time, it was running very rough, with a lot of rattling sounds from the motor. It also had a bad ass "lean hang" at 2k RPM. I re-jetted the slow jets to bigger jets, synched the carbs anad low and behold, it was perfect. Smooth idle, quick response to the throttle, returning to idle right away. At this time the neutral switch was in the "before" position, which cased the neutral light be on full time. So I took the bike for a few rides and it was fantastic. Great acceleration, easy shifting up/down.

After a couple of weeks (aside from the oil leak that I described in the other thread) I decided to fix the neutral switch side. After checking all wiring, etc, I took the switch out and had a look inside the hole, and discovered the interference with the shift drum flange. Tried to push the shift drum back into the motor with a screw driver and no luck at all. That is when I ground off the drum flange side of the N.S. a bit to look like the "after" picture and reinstalled. Fixed the N.S light problem ans it works great.

Took the bike out for a ride. Shifting still smooth, but noticed that occasionally, accelerating into a gear or decelerating from a higher gear, the bike occasionally gave a clunk and kick. It never jumped gear, as the RPMs never change, but it gave that little kick. And, it's been getting worse since.

I have turned the N.S switch around, back to the "before" configuration, but the problem persists.

Is there a way that I can push the shift drum back into the motor without having to split the case? what do you guys think the reason is for drum to be sitting a tad towards the outside of the case?

[Alan F.]

Got a very clean case from e-bay, and since the bearing colors happen to match the old case, I used the lower half of that one with the upper half of the original. This to keep the engine S/N matched to the frame VIN.

What do you guys think the reason is for drum to be sitting a tad towards the outside of the case?

I understood that mixing upper and lower case halves was not a good idea.

Engine s/n matching frame s/n indicates cases had been replaced in the past and frame number was stamped on the cases. They don't come that way from the factory.

[754]

I am wondering if you have a bent shift fork pushing things over.
 
 If you must,  remove sidecases, and pull motor , the pull bolts on top case half, then flip motor over and split cases..
 I have pulled mine one night to fix a transmission  part, and back together next night. 

[scottly]

Upper and lower cases are machined as a unit for the crankshaft mains, trans shafts, and shift drum, so it is not a good idea to mix them...
Regarding the shift drum, you may find it helps if you remove the shift cover, then operate the drum while turning the front sprocket to see if you can get the drum into it's proper position. The neutral switch fits into a groove in the drum, and limits the lateral travel in both directions, so your modified switch is now junk.

[RezaM]

I was able to push the shift drum back into the motor by wiggling the shift lever and putting a slight pressure on the flange through the neutral switch hole. The neutral switch then went in with no issue. Now since I had shaved off a bit of it previously, I inserted in with the "un-shaved" side against the motor side of the drum flange.

Started the bike, same grinding noises. Very smooth shifting through all gears.

Took chain off, ran bike for a few seconds. Still the same. Noticed that grinding sounds are rhythmic. In neutral, the drive sprocket wants to turn and I have to use a bit of pressure to stop it. But, when I do and then allow it to turn and hold it at different orientations, the grinding almost entirely goes away at a certain orientation and comes back slowly and gets at it's strongest at 180 deg from where it was  the most quite, even physically thumps against the sprocket. see video.

Any ideas why the grinding may be relative to the position of the sprocket??

[Don R]

 I suspect the clutch, it's normal for it to drag a bit but check the oil pan for broken pieces of a hardened washer, I found that in a 77K once, split the cases, located the missing washer from a diagram and got one from old trans parts.  Now it works as good as new.  It seems like it belonged on the right end of a gear cluster.

[HondaMan]

I'll bet $10 this is coming from using mismatched case halves, upper and lower. Reason: the gearshaft holes (and crankshaft bearing holes) are align-bored from one side of the case to the other. The mismatch due to the NC machines Honda used in the 1970s can be as much as 0.0008" between upper-lower halves from differing cases. The practice of replacing one half of the cases was oft tried with the K0, which broke MANY chains and cases, when cases were extremely scarce. It didn't work then, so I doubt it will work now.

What happens is: that non-aligned holes set the tranny shafts at an angle across the cases, i.e., the upper mainshaft, relative to the lower countershaft. Since the lower case holds the shifter drum, it is common for the "R" shift fork to sit closer to the mainshaft on the far end (i.e., side away from the clutch) unless the machine that bored the far mainshaft's hole (upper case) also bored the inner countershaft hole (bottom case). You may see this as burn marks on the "R" fork, in the web between the tines, if you've run it far enough to find steel fines in the oil pan or oil filter.

The gear teeth are all hog-cut, with dead-right-angle steps in their teeth: when one is set at a slight angle to the other they click and whirr over those tiny cut marks, making them whine and rumble.

[scottly]

Now since I had shaved off a bit of it previously, I inserted in with the "un-shaved" side against the motor side of the drum flange.

You have simply moved the excess lateral play in the drum to the other side! The neutral  switch body contacts BOTH sides of the groove in the drum. Note the wear line at the top of the body just under the ball; there are lines on both sides. If you can figure out a way to hold the switch in place when only rotated 90 degrees, your butchered switch might be salvaged.
 

[Alan F.]

Seems to me like you could file a new relief in the switch housing to hold it in place turned 180 degrees.

I press the LIKE button for Bryanj's reply above.

[bryanj]

You might but new switches still exist so why not do it properly

[RezaM]

OK, I did some checking in my notes, which date back to 5 years ago. I was mistaken about the mismatched case halves. As I said, tho is bike had no title, and I had to jump through a lot of hoops in our state to get it licensed. What happened was that when I took the bike to to the sheriff's office for the non-stolen vehicle verification, I took the top half of the original case with the frame in case the sheriff's office wanted to match the engine s/n to frame VIN.

To answer Hondaman's reply, sorry I led you down the rat hole brother, and I see your point that mismatched cases can cause problems, but this is not the case.

The thing that bothers me is that this bike ran super smooth for the first 50-60 miles around town while I was testing things/looking for leaks. The clutch rattled lightly only during starts until a few minutes when the bike got warmed.

This whole thing started when I shaved off the side of the neutral switch. The reason i rolled the switch around in my last post was to see if the original side towards the motor made any difference. I'm looking for clues before I split the case again. I agree that eventually I need a replacement switch.

Don R: which washers are you referring to, the half circle ones that hold the gear shaft on the clutch in the case? Oil was changed recently and the pan was clean.

[bryanj]

You dont need to split cases to fit a new switch, im sure i read somewhere the same switch is used on a honda outboard

[Alan F.]

That'd be great, newish switches don't seem particularly cheap at $60USD /50 EU. I wonder what the switch you mention is priced at?

[754]

Many of us like me have like a half dozen switches around.
 I am going to throw this out there again..
 Bearings with corrosion. . If there was any visible , or you got them clean and they sounded  ok..
 If there was ANY CORROSION. .. then  after a quite short time bearings could degrade and get noisy..
 May not be the case here but could be possible. .

[RezaM]

Well, I put the neutral switch at 90deg from where I shaved it off, meaning that either side of the switch sitting between the flanges of the shift drum should feel like the original switch. Temp solution as I ma looking for any clue that will lead to a better understanding of the problem before I split the case. I know I will have to get another NS.

Ran the bike for a few moments, and the issue is still there.

Felt around the clutch cover and it seems like it is giving a rhythmic thumping to the clutch cover (note the chrome dust cover) strongest at about the 3oclock position. It's like rapid light hammering. At this point, I'm not sure if rebuilding the clutch is a good path or not.

HondaMan, I agree that bad/corroded bearings can cause all kind of noise, but this is a thumping noise that because it's fast, it sounds like grinding. If you could put your hand on the case, especially on the clutch cover, you'd know what I'm saying. it feels like tapping that rises and falls in intensity: tatat...taataat...tatat, if you see what I mean.

Anyway, if I split the case, I will definitely replace those bearings. It's very odd that tapping can be reduced at every 180 degrees of the drive sprocket rotation. In other words, I can make the tapping almost entirely go away by holding the drive sprocket at a certain position. Maybe the transmission dogs are not meshing right?

Any sage advice based on the description I've given would be appreciated. I really don't want to spli the case and either find nothing, or find something simple that could have done.ions But I am running out of options, haha. I guess the '74 CB750 has to wait until this gets done. Thank y'all.

[scottly]

Regarding the clutch, there is a cupped washer on the shaft where the basket sits; is that washer present, and facing the correct way? Is the clutch cover the original '78 one? Early clutch covers will rub when used with a late clutch basket.
Were the snap rings removed from the transmission shafts? If installed incorrectly, the rings can pop out of their grooves.
http://forums.sohc4.net/index.php?topic=86429.0

[754]

 I was thinking, if one if the 2 final drive gears , had a broken tooth.... it may do that..

[RezaM]

Any advise on where to get reasonably priced, but good quality clutch parts would (discs, springs, etc) would be appreciated.