[bence8810]

Dear all,

New to the forum, please take it lightly on me. Also new to twins..

I live In Indonesia (Lombok) and got this fantastic twin as my first real bike. Have a few cubs etc…

i had the bike rebuilt, new pistons and all looks ok with the build.
the problem I have is that over 5,000RPM the engine starts shaking and making a little noise - hard to tell from where but I suspect the top.
Here there aren’t many bikes that are left original and this one isn’t any different. It has aftermarket carburetors and the pistons are from a C700 super Cub - brand new and block bored to match. No shop here can bore below 47mm so had no choice but to go with these type of pistons.
The bike was also converted to CDI before I got it, there is a little metal tab welded to the crankshaft just below the flywheel which actuates a pickup coil. The timing was off but I corrected it using a timing light - moved the position of the pickup coil. Timing should be ok, cam chain as well, valve gaps are good.
compression is spot on and the bike starts on one kick in the morning, pulls strong, the only issue is the vibration.
Any advice would be very welcome.
I am basically on my own with this as there aren’t other twin bikes here on the island and not one garage knows how to handle it

thanks,
Ben

 

[DD23]

Hi Ben.

C700 ?

You know of CMSNL.com for the exploded views...

Converted to cdi, does that system automatically advance the ignition with revs ?

You have a strobe signal check timing marks on the alternator rotor.

 

[DD23]

Exact model and year or find your bike on CMSNL and post a link

 

[DD23]

Also, remove plugs and look at the piston crowns. They may be a bit coked by now, but can you see any bright alloy in valve pocket area and crown periphery, made by contact with valves or head ?

Have the spark plug gaps closed up at all ?

I assume engine is tight in frame and stuff like number plate is not vibrating .

 

[DD23]

I also assume you have a manual.

Can you take a vidio to capture sound ?

Sorry for repeat of replies, internet being a pita and it's dry here today !

 

[bence8810]

Hello there, thanks a lot for your answers and here is a reply to your questions.

model: CB125K3 - 1969. I am aware of CMSNL but as so many of the parts are non-original, it is sometimes hard to work with it.

C700 piston - it’s basically a supercub C70 but a newer version. The C70 pistons until 1980 had a dome shaped crown and later models (here called C700) have a flatter crown. The piston size is otherwise the same, 47mm diameter. The original CB125K3 pistons were 44mm and here the shops can’t bore that small of a size.

The CDI does allow advancing and I saw this with the strobe light, however testing timing with the flywheel cover opened isn’t exactly easy. If I tilt the bike to the right and keep at at base RPM it’s kind of ok for about 15-30seconds, however as soon as I crank the RPM’s up, oil sprays Uncontrollably.

No shiny parts on the piston crown, there’s no touch of the valves.

valve gaps have not closed up, will recheck again today.

Is there a best way to synch the carburetors? I made sure air screws are identical in setting and also that the throttle bodies move at the same pattern on both sides.

I will try to take a video and upload.

Thanks,
Ben

 

[bence8810]

Here is a video of the timing test. I think it’s spot on now and also shows the advance working well.

Timing light video

 

[DD23]

Could you take a vidio from a bit further away please and at revs so the nouise/vibration can be heard please with bike on centre stand. Yes it will be messy with some oil loss.

I assume you can see the timing marks clearly so no over advance.
Can't see the tdc reference mark/tab on the stator. Rotor wobbles a bit ?

Intrigued about welded on crank reference ?

 

[DD23]

If you can acquire a spare alternate rotor cover, you can drill a suitable hole in it to reduce oil exiting the engine and still do the strobe timing check. You could also made a see thru window from plastic to stop all oil exiting.

Alternatively if you can get some plexiglass sheet and make a see thru cover....

As it's not often needed too check ign timing, a little oil loss and mess is tolerable.

You say no bright alloy showing on piston so no piston/valve and piston/head contact, which is good, but I wonder how the cylinder head gasket was altered due to bore size increase, perhaps a new gasket was made, from some copper sheet perhaps ???

Anyways a better vidio and sounds of engine revving to hear the noise yiu mention....

I notice a few chewed screws and nuts, hope that's not a bad sign...

 

[PoiPio]

Sounds good, fires right up, revs like right now, see no crank vibration, can't hear a peep out of the cam chain, nor a rod knock at either the big or small end, nor a piston tap swapping directions. Guess what, the more cylinders the smoother the engine.

It's a buzzer. Live with it. Any vibe it should of fallen apart already. Sounds too good to show a problem.

Signed,
NOLTT (no one listens to turtle)

 

[bence8810]

I’ve created this video, best I can do. Would need 3 hands!

I am in 5th gear going from 60km/h to 80. I hear the noise and feel the vibration but granted on the video it isn’t really showing.
Maybe check the rear view mirror how the trees are blurry. Until 60 I see everything clear in the mirror and after that it starts shaking and things get blurry. Annoying at night with the lights from behind looking like bright dots going wild in the mirror.

I’ll reply to the other questions also later.
Thanks,
Ben

CB 125K3 60-80km/h vibration

Could you take a vidio from a bit further away please and at revs so the nouise/vibration can

 

[bence8810]

Plugs as they look now after a long ride. Looks good to me but let’s see what you guys think.

 

[DD23]

What rpm is the vibration occuring ?

What sort of noise is it ?

Does it vibrate in every gear or just in top gear ?

How is the rear wheel drive chain ?

I assume stuff like tank rubbers are good and the tank is not touching the frame etc..

Plug gaps look a bit wide.

Colour looks a bit on the weak side.

Left plug thread looks troublesome ?

How is that cylinders plug hole thread ?

You have a plug hole thread cleaner/chaser ?

Be extremely carefull with that plug hole...always start plugs into plug holes by hand...

Picture of engine from a little further away, to show timing pointer on stator.....

Just to clarify, cam chain is on left of engine ?

To hear the noise at the vibration revs would be good.

You sure it's not wheel vibration or exhaust system vibration ?

From memory handle bars are rubber bushed ?

Will check on CMSNL with your bike details given.....

Another busy day for me so back much later...

Over....

 

[DD23]

Well well well, seems that CMSNL cb125k3 handlebar exploded view has a mistake.

View part 6 looks like rubber cones but in part numbers list part 6 us a steering head race cone....

I personally do not remember this model which may have been a bit rare in UK.

Pistons in phase, they go up and down together, so may have "bad vibes" at revs, no balance system other than crank webs etc..

Cb125s, a very popular single cylinder bike in UK was not really a "bad vibes" bike from memory. It also had cushed handlebars.

 

[DD23]

Well well well, seems that CMSNL cb125k3 handlebar exploded view has a mistake.

View part 6 looks like rubber cones but in part numbers list part 6 us a steering head race cone....

I personally do not remember this model which may have been a bit rare in UK.

Pistons in phase, they go up and down together, so may have "bad vibes" at revs, no balance system other than crank webs etc..

Cb125s, a very popular single cylinder bike in UK was not really a "bad vibes" bike from memory. It also had cushed handlebars.

 

[PoiPio]

As stated, new left plug. The threads are weak in the head. Plugs look perfect. How? Not carbon fouled, not oil fouled. Anywhere from white to light tan, or lean is mean gray, again, engine looks health. Can tell by the instant startup, instant throttle response are clean carbs, no coil breakdown at speed. Again, no problem chasing your tail waiting for a normal sounding, normal running Honda to blow up.

Signed,
NOLTT

 

[DD23]

Yeh but this one ain't normal, but as you say no one listens to the turtle

 

[PoiPio]

I hear you, no pun intended. I hear and see the bike. Let's run down the list of not physically riding it. All we have are video and photo. See if you agree what I see, and what you see and hear.

1. Compression:
a. Fires off fast.
b. Crank showed no vibration with timing light.
c. No oil past the rings show it looks pretty good seal to compression wise.

2. Spark:
a. I'm sure the OP took that aluminum welding rod, felt TDC, somehow meguyveered (my kind of guy) the welding rod as TDC.
b. Unless I am mistaken, that rod point is for both setting valves and matching where the advance would line up.
c. Lights off as fast as a fuel injected bike if not dead even. Can't be coil misfire and vibrate.

3. Fuel:
a. Absolutely without a doubt, that is a clean circuit in each hole of that carb(s).
b. The spark plugs will eventually turn tan, but they are low mile gray as in the best color, but we are talking looking at the bottom of the porcelain with a special plug reader, and using race gas. So looking at the top of the nose of the porcelain, that visual looks well tuned.
c. The response as if the floats are off. Not even.

Sub variables:
Those would be oil level so as to cook the piston skirts and that being 'splash lube' cooling and lubing the skirts. That goes back to the ring damage being stuck by the snowballing effect of material coming off the skirts, then the knock as mentioned.

The crank being the out of balance but is it? The initial spin up and shutdown of watching the crank thru the timing light video, I'm looking at the crank for the vibe I do not see.

Or is it that guy with the ear in the Daytona 500, and it's getting so close to the final lap, the noise that was always there is now the driver is listening, breaks concentration thinking the car is about to fail before the end.

Or is it a non-abused looking used and strange bike? And then there is the ear is not familiar with another cylinder added? And I quote, "Also new to twins.. "

Signed,
NOLTT

 

[bence8810]

Replying to your questions first:

What rpm is the vibration occuring ?
5,500+

What sort of noise is it ?
Hard to describe, it just starts knocking a little - sunds like it is coming from the top. On the video it can't be detected at all!

Does it vibrate in every gear or just in top gear ?
Every gear - yes!

How is the rear wheel drive chain ?
All good and stock. Front 15T / Rear 47T. When I bought it the configuration was F14T / R40T but CMSNL tells me it should be 15/47. I got the right sprockets from Japan and installed them along with a new chain. It drives smooth but I found this stock configuration a little too short. At 70km/h I am already passed 6,000 RPM.

I assume stuff like tank rubbers are good and the tank is not touching the frame etc..
Yes!

Plug gaps look a bit wide.
I checked and they are 0.7mm.

Colour looks a bit on the weak side.
Yes - motor runs a bit hot. Carbs are aftermarket and for a different model - this is what was suggested by a garage in Java saying this is what all Indonesian riders use once their original carb is tired. It is from another honda bike.

Left plug thread looks troublesome ?
Yes - very hard to access the left plug as it is deeply recessed.

How is that cylinders plug hole thread ?
Thread is good - new. I took the head to a shop and they remade the plug threads.

You have a plug hole thread cleaner/chaser ?
No I don't but can get one easily online. The plug sits first though...

Be extremely carefull with that plug hole...always start plugs into plug holes by hand...
Yes will do!

Picture of engine from a little further away, to show timing pointer on stator.....
The timing pointer is not there anymore, long story. In Indonesia most bikes are butchered. When this was converted to 12V and CDI ignition, they put a new coil in etc. Anyways I found TOP and market it on my video with a bent barb wire.

Just to clarify, cam chain is on left of engine ?
Yes it is.

To hear the noise at the vibration revs would be good.
I tried to record it but it wouldn't work. Engine sounds just fine on video

You sure it's not wheel vibration or exhaust system vibration ?
Yes pretty sure.

From memory handle bars are rubber bushed ?
No they are not. I am also not worried about a little vibration from the handle, it is just that the engine vibrates a lot after 5.5K and I am sure it should not be like this.

Another busy day for me so back much later...
Thank you for your time! Very valuable.

And to PoiPio's comment about crankshaft being out of balance, I think it is but maybe within tolerance? I tool a slowmotion video of it spinning so you can see how it goes out of true a little.

Video of crankshaft rotation slo-mo

And then a long overdue brief explanation of the bike.
I got it from Sumatera - another Indonesian island some 1000's of KM's away. Here in Lombok there isn't another Twin Honda from what I know, I've asked a lot of people.

When the bike was unloded from the truck in front of my house, I started it up and it ran really rough. I drove it around a few times but it was dreadful so I decided to have it rebuilt. As I find noone here in Lombok with skills, I sent the engine over in a crate to the island of Java where a so-called specialist had it rebuilt. The engine came back a month later and I had it installed in the already repainted and beautifully restored frame, just to find out it ran pretty much as poor as it did prior to sending it out.

This was the point when I decided I need to learn this on my own, I opened the engine and I found a lot of issues, parts missing (small bushes etc). I had a chance to visit Japan a few months ago and I bought a lot of second hand parts there for the bike which I then brought back to Lombok with me.
I got the bike together and it is now pretty much back to stock with all parts correctly installed. What is non-stock are the following:

CDI 12V ignition. (the coil around the flywheel is 12V / the pickup pulser is fastened underneath the left engine cover, the pickup metal tab is welded onto the crankshaft. I had this worked out pretty well i think after having played with it for weeks. I managed to get the position of the pickup to the perfect place and also got the length of the metal tab right. As you could see from the timing light test, timing is spot on now.

Pistons are from a Honda Supercub (C700) - 70cc each. Cylinder bored up to 47 or 48mm - forgot which.
Head gasket is unmodified but the piston doesn't reach as far up as it would come in contact with gasket.

Ignition coils - there are two of them, one left one right - the original had a single coil with 2 wires. Since the crankshaft in this bike is the 360degree type, both coils basically fire at the same time on every crank rotation but I find no issues with this as the piston that is not on the compression stroke will just ignore the spark - given there's no compressed fuel mixture there.

Other electronic bits, regulator-rectifier / winker relay / dynamo starter relay / battery / bulbs are all 12V but I think they are not relevant for my issue here.

The bike also overheats a little but that could still be due to the fact the new pistons only have 600KM's on them. I take it easy on the bike for now.

The bike starts with a single push of the starter button when cold, no need to play with the throttle at all. When warm, it won't start unless I give it a tiny little gas but then it starts right away.

I'll share a few photos in a second from my phone of the build - where you can see the weird crankshaft welded tab / the pickup and also the pistons in top position - not touching the gasket.

Thanks a lot again!
Ben

 

[bence8810]

Crankshaft 12V convention pieces showing. The rag is there to prevent things from falling into the engine casing as I had the bike tilted on its right

This is the piston setup in top position. The original pistons had a dome shape and would stick out from the cylinder and into the head. These are flat. I guess my compression is somewhat lower…

And a few other random build photos

Head taken apart and serviced.

New valve seats and re-used old valves - they don't leak. I went through the hassle and grinded the valves into the new seats using the grinding compound.
Photo also shows how the plug threads were reworked. They were made smaller (as the original threads were beyond repair) and I now have to use a smaller plug. The original to be used is NKG D8HA and the one I now use is C7HSA I believe. It has a little different of a heat index but I think it should be fine? Tape is used to cover up all holes for the thread repair. I also loosened up the valve rockers so no valve would be open during the repair.

New valve seals added. On this bike only the Exhaust valves use seals so that's exactly what I've done. I could not get original CB125K3 seals so I am using C70 seals - they are identical (to the naked eye)

The valve seals as seated on the exhaust ports.

I carefully adjusted all valves to the recommended 0.05mm gap.

Pistons going in.

And at the end I used the timing light (once engine was in frame and running) to make sure the location of the pickup was right. I had to move it a few times....
I also checked compression and my cheapO gauge showed 51PSI for both sides... a compression which would of course render the engine useless if was true. My gauge is far from accurate I guess but the fact both sides showed the same value was reassuring.