Author Topic: 5th gear problem???  (Read 1330 times)

Offline Johnny4428

  • SOHC Pro
  • Posts: 793
    • View Profile
5th gear problem???
« on: April 25, 2022, 12:16:53 PM »
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.😡
1952 Cymoto on Triumph bicycle.
1961 Matchless G3
1978 Honda CB550K3.
1999 ST1100 Pan European 50th Anniversary.
1975,1980,1984,1986 Honda C90’s
1973 Honda CB750K3

Offline McCabe-Thiele (Ted)

  • SOHC Master
  • Posts: 2317
  • 07505030740
    • View Profile
Re: 5th gear problem???
« Reply #1 on: April 25, 2022, 12:50:02 PM »
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.

Honda CB400 four super sport

Offline K2-K6

  • SOHC Jedi
  • Posts: 4205
    • View Profile
Re: 5th gear problem???
« Reply #2 on: April 25, 2022, 01:32:41 PM »
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.

Offline paul G

  • SOHC Master
  • Posts: 1593
  • A bit of room left in the garage what now !
    • View Profile
Re: 5th gear problem???
« Reply #3 on: April 25, 2022, 04:13:53 PM »
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  >:(
Honda CB400 4
Honda CB550 K3 (sold)
Honda CB750 UK K1
Kawasaki Z650 C2
Kawasaki Z650 cafe racer (Sold)
Honda CD175 sloper

Offline paul G

  • SOHC Master
  • Posts: 1593
  • A bit of room left in the garage what now !
    • View Profile
Re: 5th gear problem???
« Reply #4 on: April 25, 2022, 06:21:41 PM »
Diagram here
Honda CB400 4
Honda CB550 K3 (sold)
Honda CB750 UK K1
Kawasaki Z650 C2
Kawasaki Z650 cafe racer (Sold)
Honda CD175 sloper

Offline Johnny4428

  • SOHC Pro
  • Posts: 793
    • View Profile
Re: 5th gear problem???
« Reply #5 on: April 26, 2022, 08:24:57 AM »
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!


1952 Cymoto on Triumph bicycle.
1961 Matchless G3
1978 Honda CB550K3.
1999 ST1100 Pan European 50th Anniversary.
1975,1980,1984,1986 Honda C90’s
1973 Honda CB750K3

Offline paul G

  • SOHC Master
  • Posts: 1593
  • A bit of room left in the garage what now !
    • View Profile
Re: 5th gear problem???
« Reply #6 on: April 26, 2022, 10:34:00 AM »
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  ;)
Honda CB400 4
Honda CB550 K3 (sold)
Honda CB750 UK K1
Kawasaki Z650 C2
Kawasaki Z650 cafe racer (Sold)
Honda CD175 sloper

Offline paul G

  • SOHC Master
  • Posts: 1593
  • A bit of room left in the garage what now !
    • View Profile
Re: 5th gear problem???
« Reply #7 on: April 26, 2022, 10:50:27 AM »
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
Honda CB400 4
Honda CB550 K3 (sold)
Honda CB750 UK K1
Kawasaki Z650 C2
Kawasaki Z650 cafe racer (Sold)
Honda CD175 sloper

Offline K2-K6

  • SOHC Jedi
  • Posts: 4205
    • View Profile
Re: 5th gear problem???
« Reply #8 on: April 26, 2022, 11:45:44 AM »

[ Guests cannot view attachments ]

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. 

Offline Johnny4428

  • SOHC Pro
  • Posts: 793
    • View Profile
Re: 5th gear problem???
« Reply #9 on: April 26, 2022, 08:08:53 PM »
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.
1952 Cymoto on Triumph bicycle.
1961 Matchless G3
1978 Honda CB550K3.
1999 ST1100 Pan European 50th Anniversary.
1975,1980,1984,1986 Honda C90’s
1973 Honda CB750K3

Offline Johnny4428

  • SOHC Pro
  • Posts: 793
    • View Profile
Re: 5th gear problem???
« Reply #10 on: April 29, 2022, 08:19:23 AM »
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.
1952 Cymoto on Triumph bicycle.
1961 Matchless G3
1978 Honda CB550K3.
1999 ST1100 Pan European 50th Anniversary.
1975,1980,1984,1986 Honda C90’s
1973 Honda CB750K3

Offline K2-K6

  • SOHC Jedi
  • Posts: 4205
    • View Profile
Re: 5th gear problem???
« Reply #11 on: April 29, 2022, 08:44:38 AM »
This picture from early annual may help (you need the others to describe the route) in how the torque travels through the box.

[ Guests cannot view attachments ]

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.

Offline Trigger

  • Grogu
  • *
  • Posts: 7589
  • Engines built on reputation, not advertising.
    • View Profile
Re: 5th gear problem???
« Reply #12 on: April 29, 2022, 09:56:02 AM »
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  ;) 

Offline paul G

  • SOHC Master
  • Posts: 1593
  • A bit of room left in the garage what now !
    • View Profile
Re: 5th gear problem???
« Reply #13 on: April 29, 2022, 12:38:05 PM »
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
Honda CB400 4
Honda CB550 K3 (sold)
Honda CB750 UK K1
Kawasaki Z650 C2
Kawasaki Z650 cafe racer (Sold)
Honda CD175 sloper

Offline K2-K6

  • SOHC Jedi
  • Posts: 4205
    • View Profile
Re: 5th gear problem???
« Reply #14 on: April 29, 2022, 01:13:21 PM »
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.

 

SimplePortal 2.3.5 © 2008-2012, SimplePortal