Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Johnny4428 on April 25, 2022, 12:16:53 PM
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This is not good! After doing just over 100 odd miles or so after rebuild without any serious hitches I was feeling quite good about the running and handling of the bike, that is until coming back from a run a few days ago 5th gear didn’t engage properly until the third or fourth attempt. I thought that’s strange hasn’t done that before. I have removed cover and had a look but couldn’t see any problems with selector mech. Seems to change into every gear no problem spinning back wheel. But out on the road this seems to be happening every time. I don’t like the crunch of one gear spinning against the other without engaging so I’m reluctant to go out again until I get to the bottom of the problem. I’m fearing the worst which is that this is not going to be sorted from the outside of the gearbox.😡
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I would be looking at stuff like is the clutch disengaging correctly or can you / do you change up without using the clutch lever.
Whenever I have had issues with gear changes its always been either the external linkeage or in one case due to a very hot gearbox oil admittedly mostly on cars.
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If you exhamine the gearbox schematic for 750 you can see the 5th gear has different engagement and non dog type to the other ratio. It's a sliding gear that passes into a internal gear to bring the connection.
Dogs are very resilient and quite coarse in their action, but that one is different and can cause problems. Is it just not going in or does it feel like it's gone in, then spit it back out again as you open the throttle ?
Clutching doesn't in reality affect these motorcycle gearbox with the engagement the same (if you time the throttle off ok) with or without.
For interest, virtually all bike boxes are constant mesh (gear pairs run engaged even when not transmitting torque) straight cut dog engagement design.
As compared to car type (synchrmesh) whereas the gear teeth need to be spun into the same range (that's what the bronze synchronisation cones do) before then pushing the gear teeth into mesh to transmit torque.
Oddly, that fith gear may in reality go in easier on cluthless shift than with, make sure the gear lever doesn't need you to lift your foot off the peg to get full travel as if you don't go far enough it can make it awkward to get it fully home too.
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This is not good! After doing just over 100 odd miles or so after rebuild without any serious hitches I was feeling quite good about the running and handling of the bike, that is until coming back from a run a few days ago 5th gear didn’t engage properly until the third or fourth attempt. I thought that’s strange hasn’t done that before. I have removed cover and had a look but couldn’t see any problems with selector mech. Seems to change into every gear no problem spinning back wheel. But out on the road this seems to be happening every time. I don’t like the crunch of one gear spinning against the other without engaging so I’m reluctant to go out again until I get to the bottom of the problem. I’m fearing the worst which is that this is not going to be sorted from the outside of the gearbox.😡
When you striped the engine/gear box did you take the gears off the shafts.
There is one gear that you can put back the wrong way around and it will cause this issue of it jumping out of fifth gear.
It doesn't look different until you look very carefully.
I will look for more info tomorrow but if you search on the US site for fifth gear problems there is a whole section on it.
Don't as me how I know this >:(
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Diagram here
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Thanks guys for your input much appreciated. I have had the bike up on the lift going through the gears and it seems to be doing what it is supposed to do engaging all the gears so I suppose it must be spitting 5th gear out. I haven’t been back out for a run to try changing gear 4 to 5th without clutch. I didn’t have the gears stripped from shafts completely but I did have some off as I replaced worn bushes and bearings. So at this stage I wouldn’t rule out the possibility Paul. I think I’m “going to do what a man’s got to do” . Good job it’s not nice riding weather!
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Thanks guys for your input much appreciated. I have had the bike up on the lift going through the gears and it seems to be doing what it is supposed to do engaging all the gears so I suppose it must be spitting 5th gear out. I haven’t been back out for a run to try changing gear 4 to 5th without clutch. I didn’t have the gears stripped from shafts completely but I did have some off as I replaced worn bushes and bearings. So at this stage I wouldn’t rule out the possibility Paul. I think I’m “going to do what a man’s got to do” . Good job it’s not nice riding weather!
You can split the cases with the head and barrels in place ;)
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Is this any help to you from over the pond but quite comprehensive and Hondaman is involved in the thread.
http://forums.sohc4.net/index.php/topic,138663.0.html
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[attachimg=1]
The manual picture may help you.
The last two images of 4th then fith gear selected show the gear moved into contact to get those two. Right for dog engagement, left for gear teeth engagement and quite a long throw for the selector fork to get from one to other. I always think it's the most imprecise shift in that box, and got to be certain you move it competently to be clean.
If the gear teeth on the smaller gear (going inside that far left cog) have any significant rounded noses to the teeth it can easily push it back out again under torque.
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Thanks for the info and pictures guys. Holy that’s quite a read Paul. A lot of stuff to digest, I have enough info to try one or two small things before I go in head first. I am now very intrigued myself to find out what the problem is. That’s a very handy diagram with the gear positions to help visualise possible problems.
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Having read quite a bit about 5th gear problems this last couple of days I have a better understanding of the problems that could cause this issue. In particular the amount of insertion between gear 2/3 into gear 5. Shim on the outside could cause a problem or even the lack of. Also selector fork wear which was checked when apart. Paul I get what you are saying regarding gears fitted incorrectly, but I am failing to grasp how these gears in particular you are referring to could effect or impede the selection of fifth gear. I’m probably being stupid (wouldn’t be the first time) so can you enlighten me.
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This picture from early annual may help (you need the others to describe the route) in how the torque travels through the box.
[attachimg=1]
Certainly not being stupid, I study these things and working through think aha I've got it, only to then think is that really right :) and then go through it again.
This picture, the gears with grey shading in their structure all spin free on the shaft, with the "clear" type engagement through splines. To get a spin one to connect to the shaft you need to slide along a geared one and connect the two together.
In this case the gear @ 3 spinning free is connected by moving the gear pair to it's right into engagement with it (that's the fith gear connection) then it can transmit torque coming in @1 through it's pair on the other shaft (splined) along that secondary shaft handout @ 2 (both those last two gears being permanently splined to that second shaft) all other gears during fifth engagement (same for any gear selected) just roll around with no connection through them (thats the constant meshing part) so it should just be that single fith gear movement (top left pair in this photo) that's affecting the ratio.
As mentioned, overlaying the route picture details onto this photo gives the full picture.
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You can not go by any manual when it comes to the 750 gear box as Paul found out. The manual will not show all the washers and which way they go round. Some of the very early boxes had extra shims.
The big mistake owner make is not locating the natural switch. Before you put any cogs in put the selector drum in and then the natural switch.
I will let Paul own up to the mistakes he made with going by the manual ;)
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You can not go by any manual when it comes to the 750 gear box as Paul found out. The manual will not show all the washers and which way they go round. Some of the very early boxes had extra shims.
The big mistake owner make is not locating the natural switch. Before you put any cogs in put the selector drum in and then the natural switch.
I will let Paul own up to the mistakes he made with going by the manual ;)
What mistakes what manual ;D ;D Trigger did tell me to burn it to keep warm ??? so I did.
On the reply further up I don't know why those gears the wrong way affect 5th but they do as Hondaman explains.
Trigger did come to my rescue ;D
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The assembly of it may be a debate, but the shematics above do give good overview of the routes through the gearbox.
It's the same for any gearbox though, and particularly in these dog boxes, that the free spinning gears (the grey ones in large picture) must absolutely be located correctly as any unwanted thrust direction movement can bring a compromise in how it works. Too much float and they'll bring more than one gear into engagement and start taking lumps off each other.
Conflicting gear engagement is effectively what they are using in the sesmless change race boxes. As an example;- if the box is in 2nd the change to third puts that one in use while second is still torque loaded (the shift forks operating independently) then as the 3rd gear takes up torque the 2nd gear dogs are out accelerated, just after that and before it trashes itself the 2nd gear engagement is pulled to avoid this. The torque gets handed from one ratio to the other without any real interrupt, adjustment in milliseconds and engine torque mapping is used to optimise how this is managed.
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Thanks guys for diagrams and info, it’s unfortunate that I seem to know a lot more about it now than I did when it was in pieces. But the reality is I tried just to put back together as I found it to the best of my ability, albeit with some new bearings and bushes. Maybe have a go this weekend to take engine out and open up, at least I’m armed with a bit more knowledge goin in this time.
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Just an update on findings. There seems to be excessive end float on 5th gear which must have added or even been the cause of this problem. (At least 2mm) This because of the low insertion from 2/3 plus a little wear was allowing the gear 2/3 to be thrown out of 5 and jamming between selector fork and gear 5 causing a bit of heat as can be seen from photos. So that has left me now probably having to replace gears 2/3 and 5. Also have to look for shims to pack out 5 from case.
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I know yours is a different model but on my 400 I had an issue with what felt like excessive end float on the gear shafts. I was worried about adding extra shims in case it led to a locked up gearbox when the engine was hot.
I ended up changing the outer half clip bearing retainers and fitting some new outer needle earrings plus the locating pins. When I bolted the Crank case halves together I made sure I put some Hondabond on the outer races to keep the keep the shaft float at a minimum. Remains to be seen if its worked long term.
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I wouldn’t be frightened to take up most of the end float on this Ted, but I would need to find out how much end float is supposed to be there.
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At least you've got a diagnosis now and certainly worthwhile taking it apart again.
I can never resist changing gear on boxes after putting them together to see what they feel like before running them, just a fascination in how they work really. You can pick up reluctance at that stage so suppose it helps.
Went to look at a Kawasaki Z1000 (from memory) that a friend's friend had difficulty with as it would only select first gear. He'd had the motor out to paint the frame without working on the engine, put it back in and found this non selection, then pulled and stripped bottom end but found nothing. When we went through it and found a neutral finder device that was supposed to do this, three ball bearing in arrangement that flung out from centrifuge when the output sprocket shaft rotated. No road speed and only first or neutral available, nothing wrong at all. Hadn't seen one at that time as Honda didn't use them.
Hopefully you'll get this one back to scratch again without big costs.
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The Bluing is a lack of oil. Are the oil holes lined up on the shaft with the cod hole ?
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Not sure Trigger I will have a look I never had these apart.
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Not sure Trigger I will have a look I never had these apart.
The early cog did not have holes but later ones did. You need to check that no oil ways are blocked in the gear box.
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Will do Trigger I will have to strip right down and thoroughly clean after the grinding of the gears, I will check all then.
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Stripped the main shaft gears this morning, here’s the damage. Don’t know what’s happened with the 4th gear circlip. (this must be man made damage) But I am sooo glad I have stripped the shaft this time. Oil ways were lined and there appeared to be copious amounts of oil about the components.
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Stripped the countershaft this morning all seems to be fine with all gears orientated correctly and circlips fitted correctly. Incidentally there is very little difference either side of the final gear. The diagram suggests that it is flush on one side, this is not the case of course, probably just to illustrate that there is a difference. I make it about 0.1mm of a difference between the sides.
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Went to look at a Kawasaki Z1000 (from memory) that a friend's friend had difficulty with as it would only select first gear.
Zed forums are full of people, over the years, who can only get first after a rebuild, as you say its a safety feature, to only let you get first or neutral when stationary. Why that makes it safer, I don't know. Some pull the engine apart again, only to get the same result of course. I can't say I'm a fan, I wanted to check my Z1A gearbox worked OK before I put the engine in, not wait until I was on the road again.
When I rebuilt my CB750 K1, I remember being paranoid about whether top gear was in the right way round, and deeply regretting not keeping the cluster together, as I was damned if I could tell which way round it went, although I knew it mattered. I must have guessed right as it worked fine, but I did worry.
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Went to look at a Kawasaki Z1000 (from memory) that a friend's friend had difficulty with as it would only select first gear.
Zed forums are full of people, over the years, who can only get first after a rebuild, as you say its a safety feature, to only let you get first or neutral when stationary. Why that makes it safer, I don't know. Some pull the engine apart again, only to get the same result of course. I can't say I'm a fan, I wanted to check my Z1A gearbox worked OK before I put the engine in, not wait until I was on the road again.
When I rebuilt my CB750 K1, I remember being paranoid about whether top gear was in the right way round, and deeply regretting not keeping the cluster together, as I was damned if I could tell which way round it went, although I knew it mattered. I must have guessed right as it worked fine, but I did worry.
You can usually spin bike gearboxes with the first/mainshaft before putting the clutch on so that you can check the gearbox all works without any reservations before committing to final installation. Old trick on things like Triumph Dolomite sprint and Rover V8 engine when you could remove the distributor and using ad-hoc quill shaft in electric drill to spin the oil pump and bring oil pressure up prior to running springs to mind.
You may be able to spin the Kawasaki input shaft fast enough by hand to check changing upward, or make a drill adapter to spin that shaft and facilitate it that way.
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Johnny4428, was that circlip installed like that with the tang sticking out ?
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Johnny4428, was that circlip installed like that with the tang sticking out ?
I’m guessing Nigel that this circlip was a victim when the bike threw the chain and it came through the crankcase. You can see an impact mark on the side of it.
Also centre shifter fork a candidate for replacement.
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She’s back! All gears present and correct. Had my first run yesterday after fitting exhausts, tank, seat and side panels setting clutch and filling and priming with fresh oil. I must say I have a much better appreciation for the mechanics of the gearbox. I think I will be tentative changing to 5th gear for some time to come until confidence returns. I replaced gear 2/3, gear 5 and centre shifter fork that were all damaged due to gear not engaging properly as well as all circlips. Short run yesterday as near side mirror came loose and idle was too high after checking bench sync on carbs. Thanks to all for your input. 😊😊😊
By the way I used the 4”x 2” method of engine removal and refitting, worked a treat.
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Good to read its sorted in what to me looks like a relatively short time in my time warp world.
Must be a great feeling to have sorted it - always good to read a positive final outcome.
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In the unlikely event that it happens again, I will be running with a 4 speed Ted. 😂😂😂
Looking forward to hearing of your run when health and weather prevail.😉
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In the unlikely event that it happens again, I will be running with a 4 speed Ted. 😂😂😂
Looking forward to hearing of your run when health and weather prevail.😉
Can't see you would be happy with a gear missing for more than a nano second.
I'm driving Wendy mad when I start verbalizing thoughts I should keep inside - such as - "it might be safer to ride the bike with my stitches still in place as that would be stronger than after they have been just removed " Wendy's reply would not get through the censors.
When friends ask me how I am I say "still can't ride the bike yet" rather that say "I'm still sore with an angry looking scar with yellow bruising surrounding it."
I find it pleasing when I look at the 10 day weather report with 5 days of rain that somehow eases my frustration.
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My usual answer to that question is still breathing
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I like that one.
I also like "Still waking up with a pulse" as it it's a question as well.