SOHC.co.uk Forums > CB750
5th gear problem???
Johnny4428:
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.
K2-K6:
This picture from early annual may help (you need the others to describe the route) in how the torque travels through the box.
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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.
Trigger:
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 ;)
paul G:
--- Quote from: Trigger 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 ;)
--- End quote ---
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
K2-K6:
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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