Honda Cg125 noise/vibration from output shaft?

[moto42]

Hello all, first post on here, nice to meet you all. As the title suggests i'm in need of a second opinion on what (I think) may be an internal transmission issue on my 1990 CG125.

I first noticed some vibrations whilst riding, mainly towards the higher end of 3rd gear, when engine braking at certain speeds and when accelerating over 50mph, it is rythmic and can be heard / felt as a 'thunking' when doing slow first gear maneuvers. I noticed it to be at the left hand side of the bottom of the engine, as I could feel the vibration the most in the left foot-peg, although it seemed to travel all the way to the end of the seat. After checking and adjusting chain tension meticulously, I started Thinking it could be a binding chain or worn sprocket, I changed both the sprockets as well as the final drive chain. Whilst doing this, something which confused me was that the sprocket and transmission would feel smooth when being turned with the chain off, no major vibration or noise. My best guess at the moment is that an overly tight chain has caused some wear on a bearing/something within the engine and putting the chain on causing more pull on the output shaft?

It got weirder when I noticed there were tight spots in the brand new chain I put on the bike, even after making sure the rear wheel was aligned. After trying riding the vibration was still noticeable, and thats when I started to think it to be a more internal problem. I have attached a video of me spinning the rear wheel by hand, as I do this you can hear both the normal, higher 'tick tick tick' of the front sprocket making contact with the chain.. But also a deeper, 'thunkier' noise, which can be heard (as best as I can locate) behind the front sprocket.

I'm hoping I could get a second opinion on whether I am right in thinking I should strip down the engine and inspect the transmission and bearings for wear? It is slightly annoying considering the engine only has 3500~ miles on it, but all I can guess is that due to it being my first bike, it has been caused by my user error.. (chain too tight, dragging clutch?) I have already got the engine out of the bike in preperation to do so, I was told it would be easier to just replace with a different engine but doing so feels wrong, plus it's a chance to become more au fait with my first bike.

Any help would be really appreciated, specifically surrounding the nuance of the vibration / noise being present with the chain on (and at any tension) but appearing to come from behind towards the transmission.

VIDEO: (TURN SOUND UP, HEADPHONES HELP)
https://youtube.com/shorts/6R9vbvZ9M7g?feature=share

[blurredman]

I'm sorry, I know the bike is in Neutral in the video- but does this noise occur only in a specific gear whilst actually riding?
____________________
CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S, 1979 MZ TS150.
Current: 1973 MZ ES250/2 - 18k, 1979 Suzuki TS185ER - 10k, 1981 Honda CX500B - 91k, 1987 MZ ETZ250 (295cc) - 40k, 1989 MZ ETZ251 - 51k.

[moto42]

No need to apologize, appreciate you going over it in your head The vibration seems to happen throughout all the gears, just at different rpms / 'times'..

For example in first when slowly creeping, It turns almost into a thudding. In second and third it starts towards the end of the rev range. But when in 3rd or 4th at certain cruising speeds the vibration is quite unbearable, and will persist until the throttle is released or I rev out of it. Past a certain speed in 5th the vibration is un-escapable.

It's hard to pinpoint when exactly this started happening but I presume it's been after one of the longer journeys I've been on, possibly with the chain too tight..

[A100man]

I would expect that 'too tight chain' would only really affect the output shaft bearing and that possibly you could detect that by 'waggling' the output shaft/front sprocket.

Other internal woes could be the result of low oil, although the engine would normally seize top end first before affecting the g/box bearings I reckon.

A 'new' engine is usually suggested due to the hassle of stripping/rebuilding ad the cost of parts. But OTOH its 'only' a single so you could give it a try if your interest takes you that far.
____________________
Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[blurredman]

Take the sprocket off and the bearing oil seal cap (if it has one?) and inspect physically I would say. Indeed tight chain can cause problems. If you're lucky and this is the only result in that potential situation, then that's good. Better the bearing giving you warning than just shattering and then all the internals twisting.
____________________
CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S, 1979 MZ TS150.
Current: 1973 MZ ES250/2 - 18k, 1979 Suzuki TS185ER - 10k, 1981 Honda CX500B - 91k, 1987 MZ ETZ250 (295cc) - 40k, 1989 MZ ETZ251 - 51k.

[moto42]

[moto42]

[stinkwheel]

Does sound a bit like a grumbly bearing on your video.

There are normally some tight spots on a chain, but if it seems excessive, a bent mainshaft is also a possible. The shaft is splined so it would be tricky to measure any runout using a dial guage indicator unless you sleeved it. Although you could check the clearance at the top of the splines if you could get a firm anchor. I guess another way would be to remove the sprocket and run the engine in first gear and look for any wobble on the end of the shaft. Maybe with a wire "pointer" set to just off the top of the splines.

If the bearing was shot enough to cause tight spots, I'd expect there to also be play on the shaft.

The bearings themselves will be cheap, I think it's a 6204 so about £20 from Honda, about a fiver from a bearing factors for a premium brand.

Downside is vertically split crankcases so you need to pretty much take the entire engine apart to get at it so add a full gasket set and a good few hours work to the equation. Along with the use of a couple of special tools to get the rotor and clutch hub off. On a more modern engine design, you could probably turn it upside down and sneak the bottom half off the crankcases with a lot less stripping.

Alternative is to wait and see if the situation develops. If the bearing is on its way out, a grumbling will become a rumbling will become a knocking then a catastrophic failure.

EDIT:
For reference, this is the output shaft bearing on a wee 2-stroke I rebuilt recently. It apparently still ran and rode when it was put away in the corner of the shed but with some utterly horriffic transmission noises and a massive gearbox oil leak.
https://lh3.googleusercontent.com/pw/AP1GczOHMhGSB4qhFuxkCTPdSQQMYSRdXuq2_Npgxz2zPxA94rfYuESKsbJcWHKQxp7NUjFUl3BT2oU2D8GS1P76mlh5hBjpQn9pJQFH_lo5iKqC8ECYswXF3HRVazlI9ANy0Ze_9e32vVZHqIyh8R_VVNBy=w1669-h939-s-no
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[moto42]

[moto42]

[A100man]

It doesn't sound great does it? if it is a bearing then I think it's truly nadgered. Can you drop teh oil and then go fishing with a magnet?
____________________
Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[stinkwheel]

Given how much work it is to get to, I'd be using a high quality bearing.

I'm sure wemoto are fine. I get mine from simply bearings or a local bearing factor where I'd expect to pay £5-6. As I said, I think it's a 6204 but you can confirm that by measuring it or reading the bearing code stamped on the outer race. If it's a 6204 it'll be 47mm OD, 20mm ID and 14mm deep. Can't think you'd want anything different than N (standard) clearance for an output shaft. Open deep groove ball bearing.

I generally go with SKF, NSK, FAG, Koyo or timken on the basis I don't want to do it again. It's not a place to save money. The difference between a good one and a cheap one is about £3. Thay also sell oil seals on simply bearings.

I highly doubt the gaskets for a 90s japanese motorbike are asbestos.

Remember the shaft that bearing is on has a second bearing on the other end, if one's gone due to eccentric loading, I'd be tempted to do the other one too while I'm in there.

They are simple engines, a good first time one to work on. Take lots of photos, make sure you have a good manual. The haynes type are more beginner friendly, the actual Honda ones give you more information if you know how to look for it. Particularly regarding order of operations, do the things in the order it presents them. For example to get the rear shock off my VFR, it tells you to remove the seat, disconnect the battery and dismount the starter solenoid. I read that and was "What the hell? Surely you just need to take out the top and bottom bolts." After half an hour buggering about not being able to get it out, I went back to the manual and found there's a hatch in the bottom of the battery box it lifts out from.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[A100man]

[moto42]

[moto42]

[A100man]

[moto42]

[A100man]

WD40 should help dissolve any sealant or gunk, if not maybe acetone.
First make sure you haven't missed a bolt or something else that could be snagging it,
____________________
Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[stinkwheel]

I never worked on a CG125 but once you're absolutely certain you have all the nuts and bolts out, there is a trick I've used before to get a "stuck" barrel off.

This is potentially quite a hazardous thing to do so at a minimum wear hand and eye protection, I preferr a visor which also covers my neck area.

The only thing that should be holding it is either a very tightly "glued on" base gasket or corrosion in the stud holes. Get a packet of stanley knife blades and a small hammer. With care, you can insert the blades into the paper base gasket at several points and start tapping them in. Eventually they should start to split the base gasket in half and begin to wedge the barrel up off the cases. If it's just the gasket, it should eventually let go. If it's corroded studs it'll start moving up to the point you can get a second one in then use some sort of tool (a putty knife works well) to lever in the gap.

Stanley knife blades are made of very hard steel so hitting it with a hammer risks shattering them and small pieces of very sharp settel flying off at high speed. Also be very careful not to damage the soft alloy of the crankcase gasket surface so if your barrel is already broken, aim more in that direction. If levering leave the bottom blade in flat to protect that surface and lever against the barrel.

EDIT: At this point since you're splitting it anyway, there would be no harm in loosening the crankcase bolts in case the cylinder liner is binding against the crankcase opening.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[moto42]

[moto42]

[moto42]

[stinkwheel]

No harm swapping them out for all they cost.

Do you know anyone with a lathe? Be interesting to get the mainshaft between two centres with a dial guage on it to check for runout. Given your reported tight spotting of the chain.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[jeffyjeff]

[A100man]

Im a bit out of my depth here, however I'd suggest leaving the bearing to start with and part reassembling the gearbox only - a bit at a time -until you can identify the true source of the grumble.
____________________
Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600