Gearbox problem

[taysidedragon]

Looking for advice on a gearbox problem I have with my 400f.
When rolling to a halt in first gear with the clutch pulled in there is a clunking sound  which appears to come from the clutch / gearbox area. It feels as if a gear or shaft is jumping out of line. It doesn't happen in any other gear and gears change up and down without any problem.
I've drained the oil and checked filters, there are no metal particles or evidence of damage. I removed and inspected the clutch and it all checks out within spec.

Is there a common problem with gearbox wear? I guess I'm going to have to split the cases to find the problem. 

[Bryanj]

No common problem i know of; is the chain nice and free on the links as i have seen bad chains cause odd noises

[Nurse Julie]

Had a seized link one time  on the Thundercat drive chain that used to make a terrible noise. If its only happening in 1st gear with the clutch in, I would think its something catching somewhere 🤔

[MrDavo]

I had a strange and puzzling intermittent clunk with my CB750, which I could only hear (and feel) at slow speeds, like 'Clunk'.....................'Clunk'.....................'Clunk'

It turned out to be one of the studs holding the rear sprocket onto the hub, was slowly backing out, with the nut on, and being machined by the inside of the swinging arm.

[taysidedragon]

I'll check around everywhere for the above issues. It's a fairly new chain and sprockets which have only done about 400 miles, but you never know.

[sye]

Is the chain run straight? Check it with a plank or better still a laser tool. If it's off by more than a little, the chain catches on the front sprocket sides and you would only hear it when pushing the bike or moving slowly.

[ka-ja]

Or maybe a problem with the primary chain?

[florence]

I'm assuming from your original description that it doesn't do it in neutral?

[flatfour]

My friend had a similar problem with his 1970 CB750. It came on quite suddenly and could only be heard when slowing almost to a stop, when it was loud enough to notice from another bike if you rode slowly alongside him. At this time he had owned the bike for some nine years and the total mileage was around 35,000. No work had been carried out prior to the problem developing.

The engine unit was dismantled and rebuilt, with no faults evident. After this though, the problem disappeared and did not re - emerge in the following 6,000, or so miles that passed under its wheels before it was sold!

In addition, a CB750 K6 that I owned and then sold to the same person mentioned above developed a rapid "tick" from the chain case area after a new "O" ring chain was fitted in place of the original. This however was easily solved, as the new chain was simply too wide and a replacement non - sealed chain immediately put matters right.

[taysidedragon]

I'm assuming from your original description that it doesn't do it in neutral?

I didn't click it into neutral while still moving so I don't know. Can't check it now because it's in pieces.
I've double checked the chain and sprockets, nothing touching or clicking, no sign of any contact anywhere. There seems to be a lot of in and out movement of the gear change shaft, a lot more than on a spare engine I have. 😯
If there was a worn primary chain then I would get the clunking in 2nd or 3rd gear when slowing down, wouldn't I?
Either way, it looks like engine out to investigate. 😣

[K2-K6]

I'm not that familiar with the internal gearbox operation of the 400,  but if you have lateral movement on gearshafts or selector drum then it's possible that while in one gear it would be rattling the dogs on another ratio.

Not enough to give you a complete conflict (if it did it would break dogs or teeth) but just skimming another ratio while in first would certainly make a noise.

Given the above,  you'd have to consider if you've got a damaged tooth on one gear, that could be making a noise during torque reversal of the rear wheel driving the engine under braking.

Does look like you need to get in there for a more comprehensive look though.

[florence]

Before taking the engine out you could always have a look in the clutch casing.  I once had the end of the selector drum come loose and it caused all sorts of problems and strange sounds.  This casing can be removed with the engine in the frame.  The spring on the selector thing at the end of the gear-shaft might also be a problem.