SOHC.co.uk Forums > CB750

5th gear problem???

(1/8) > >>

Johnny4428:
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.😡

McCabe-Thiele (Ted):
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.

K2-K6:
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.

paul G:

--- Quote from: Johnny4428 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.😡

--- End quote ---
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  >:(

paul G:
Diagram here

Navigation

[0] Message Index

[#] Next page