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CBF125 Overrun Rattle

Blog Date 23 August 2020

The long suffering CBF125 still runs and rides well enough. At over 90,000 miles though it has just developed a new problem. I can ride and ride and ride without issue. Then if I've "given it large" over the big hill hereabouts something unsettling happens on the overrun coming back down the hill. 

I'm sure 99% of you know what overrun means - but just in case. Overrun is when the throttle is closed and the engine is being pulled around by the momentum of the motorcycle rather than the more normal state of the engine pushing the motorcycle forwards. Engine braking happens during overrun.

Coming down the steep hill on the overrun there comes a terrible horrible rattle that is entirely "in sync" with the revs. I'd say it was in sync with the combustion stroke (ie half the crank speed) but my ears aren't THAT accurate. 

On first hearing this I thought I'd blown the head gasket and the noise was the air actually being sucked into the motor. EH? Sucked? Engine braking occurs NOT due to the compression stroke but due to the piston drawing down HARD against the closed throttle valve on the induction stroke. With the throttle closed at high revs little air is drawn into the piston so compression would be low anyhow.

Expecting to have no power on the next flat section due to the blown gasket I was surprised to find the bike continued as if nothing at all had just happened. Confusing. After a bend the next downhill also induced the same horrific clatters only to be followed by normal service. Odd.

I used the bike for local runs (5-10 miles) without trouble then this weekend I had ridden 40 uneventful miles until I reached the big hill on my way home again. Same again, give it large up the hill, over the crest and RATTLE! Same again on the overrun, same normal service on the flat sections.

So, what could it be?

Head gasket? It has developed an oil leak now but if it were leaking (badly) from the combustion chamber I would expect to at least hear it under power as well as on the overrun. Power output is within normal expectations too.

A small oil leak from the cylinder head gasket on the CBF125
This is the oil leak... in, erm, err. Oh dear. 2016. That was 32,000 miles ago.

Big End/Mains/Small End? Perhaps. The problem with this theory is I would expect the noise to be present more often, all the time even. It has taken a very particular set of circumstances to bring on this noise (hot after an uphill blast). My own limited experience and online searches suggest the noises from these issues would be omnipresent.

Gearbox? No, this rattle is entirely in sync with engine speed not road speed or shades inbetween. 

Exhaust Leak? Again I'd expect to hear this under load rather than on the overrun. Unless... unless it's "popping" from unburned fuel much like a 1970s sports car (or modern "yoof" car with specially mapped fuelling to create the popping). No, no it's not a popping sound and popping tends to be intermittent rather than exactly inline with engine speed.

Which leads me to my favourite but unconfirmed theory. Camchain/Tensioner. This theory much like string theory is not without its holes though.

Imagine your final drive chain. Under normal forward running the top section of chain is under tension and the lower run is "slack". On the overrun as the wheel is driving the engine the lower section is tight and the upper run is slack. By the way I have considered the final drive chain to be the issue and checked this. 

Diagram showing which section of chain is under load on drive or overrun
The final drive chain's taut section changes from being under power to throttling off.

Inside the motor the camchain has a simple guide on the "tight" side and the tensioner is on the "slack" side. What if, like the final drive, on the overrun the normally slack tensioner side is pulled taught? With a worn camchain and tired tensioner the tensioner and/or guide are loosening off allowing severe camchain rattle?

The camchain and tensioner setup in a simple diagram
The camchain is just like the final drive...right? Erm, not quite.

But wait! Unlike the final drive the crank pulls the camchain and thus the cam itself. It's easy to see how the section on the final drive chain can come under tension under load and then slacken on the overrun. But there is never a time when the camshaft can pull the crank, there is never a way the tight section of the camchain can become the slack section. There is no method for the cam to drive the crank.

Dagnammit! There is a massive hole in my theory. And yet, yet, somehow I "feel" the camchain tensioner is a good candidate. 

Why? On Sharon's Kwakker 250 the tensioner works thusly. A spring pushes against the tensioner blade providing tension. However this spring is on a one way ratchet. If the camchain slackens then the tensioner tensions but it cannot then be pushed back. 

The ratchet camchain tensioner as a simple diagram
Due to the one way ratchet the tensioner can only tighten, not loosen.

On the CBF125 (and the CBF250(CBF 250 Cam Chain Rattle - SOLVED!)) the tensioner is essentially still a spring but relies on the friction of a thread to stop itself being pushed back too easily. This, this is quite frankly poop, crap, stupid and daft. The ratchet system works. It is simple. Honda uses ratches in it's larger machines. Why oh why create a stupid friction system when they have access to perfectly good systems on larger machines? I love Hondas but they are far from perfect.

Admittedly this friction system has worked for 90,000 miles. But as the spring is likely tired, the threads likely old and the camchain likely worn I am pondering if this overrun rattle is the first signs of things giving up? 

I know, understand and accept I am clutching at straws. I am open to the wisdom of the masses. As I write the cylinder head and barrel are off the CBF125. There will be another address forthcoming regarding the interior condition of the piston, barrel, cylinder head, valves, crank and camchain. 

In the meantime folks mumble a prayer to the gods of the infernal confabulation engine, pray that I remember how to put it all back together.

Honda CBf125 cylinder head
Oh poop, it's never going to run again is it.

If you'd like Ren to test ride and review your motorcycle contact ren@bikesandtravels.com

Reader's Comments

Upt'North said :-
My first question is always.......well probably always.
What did you do last on the poor neglected thing? Check that first.
But it's tired and like all tired things it should be allowed to sleep or shoot it with a shotgun.
I'm thinking cam chain and yes some of the earlier Honda adjusters were stupid too. Never ever follow the Honda manual.
Good luck.
Upt'North. 0
24/08/2020 11:24:39 UTC
Ross said :-
It's a Honda, it'll be the cam chain or tensioner...now what's the question? If you're sure it's not doing it at any other time I'd say the cam chain tensioner is the prime candidate...is it one of those rubber/plastic slipper type things? Hopefully it isn't breaking up and sending bits all around the engine! Have you tried applying 'black gloop'? I hear it fixes most things! ;)
24/08/2020 12:17:08 UTC
ROD said :-
My advice is to get better ear protection so you do not hear this noise.
Problem solved!
24/08/2020 02:50:18 UTC
Bogger said :-
Can you not remove the tensioner strip it clean it and put it back in. Replace the timing chain as well. Wemoto sell them for £36.00 and if you're feeling extravagant a blade for £29.00.

Under £70.00 and your problem is solved.

Go on get yer purse out man.

24/08/2020 05:13:39 UTC
Snod said :-
I'm going with the exhaust, especially if it's one of those small-downpipe-inside-a-larger-downpipe thingers.
24/08/2020 05:48:16 UTC
Ren - The Ed¹ said :-
I have the head back on... so far so average.
24/08/2020 07:24:50 UTC
Bob said :-
Camchains do tend to rattle on the overrun.
When was the camchain last changed?
The KLX needed a new camchain at 13000 miles and a manual tensioner. The inadequate automatic tensioner was the reason the original chain had failed. 20000 miles and counting on the replacement and I've only had to adjust the tension a handful of times.
I guess you've checked the final drive chain?
26/08/2020 07:34:06 UTC
Ren - The Ed¹ said :-
I do agree Bob but what I can't understand is the WHY? As explained it makes sense on a final drive as the normally slack lower run becomes tight when the wheel turns the engine rather than the normal engine turning the wheel. I the crank is also slowing perhaps quite rapidly but the only possible way the cam can turn the crank not the other way around in inertia in the cam... But the cam is tiny and greatly slowed by the work of opening and closing the valves.

I have checked the cam chain now and decided "it's OK". I have created my own manual tensioner and I estimate it is roughly half adjusted. If the problem persists I may fit a new chain as (mercifully) the crankcases do not need to be split to do this. It is a comparatively easy task now I have a flywheel puller.

Prey tell, did you have to split the cases on the KLX? As Sharon's bike is the same engine it'd be nice to know. Sharon's cam chain and tensioner appear to be fine. That said in 25,000 miles she's also not used ANY brake pads, or any material from the original pads for that matter. We've adjusted the final drive chain once. While being a hobbit limit's motorcycle choice greatly it does seem to greatly reduce the wear on the bike.
26/08/2020 08:39:18 UTC
Bob said :-
Yes it's odd about camchain rattle on the overrun. It is down to rapid engine deceleration and maybe on the overrun you can just hear it because there isn't the normal exhaust noise to drown it out.
I've never seen a single cylinder engine that needed a case split to change the camchain, by definition it can't be buried between cylinders so it has to be accessible.
On the KLX it's clutch cover and basket and primary drive cog off then loop the new chain around and reassamble.
On multi cylinder engines I always just use a split camchain and soft rivet link - never had any problems.
FWIW a manual tensioner increases chain life because it doesn't actually place the chain under tension, it just takes away the slack. The automatic types place a constant spring load on the chain.
27/08/2020 09:04:24 UTC
Mark Noel said :-
I vote for clutch plate rattle: the direction of slack will alternate between drive and over run, hence leading to a rattle. Failing that it could be a loose battery in your hearing aid moving forward during negative G man-hoovers.
27/08/2020 10:40:52 UTC
Upt'North. said :-
"Loose battery in your hearing aid", I'm off to the k. k. k. kitchen Granville to f. f. fetch a cloth, for the tea I've just spat all over my tablet screen.
27/08/2020 10:48:10 UTC
Ren - The Ed¹ said :-
Cheers Bob, useful to know. It would be "possible" to put the camchain inside of the crankcases and thus inside of the main bearing but on reflection that would be a really silly design would it not. Interesting re the manual v auto tensioners. I can see the logic of what you're saying and yet auto tensioner are to the best of my experience, everywhere. Considering most camchains way outlast the warranty and most likely the engine it's just easier to fit auto. It's only folks like you and I doing silly mileages that wear them out.

Mark Noel - Thank you for applying your well established superior intellect and wisdom on this subject. Your contribution is, erm, errr, welcome. GIT. I'm glad to see you using the modern spelling of manoeuvre too.

Upt' - It's not just me then with the tea issues on this website.
27/08/2020 02:11:07 UTC
Bob said :-
Manual tensioners are the best IMO.
Wander around a race paddock and have a look - the vast majority of bikes have manual tensioners.
The reason manufacturers fit automatic tensioners is that the average owner can't be trusted to properly use a manual one and the consequences of incorrectly setting one up can be engine failure.
27/08/2020 06:19:39 UTC
Ren - The Ed¹ said :-
Bob - what do you consider the correct tension? Previously I have turned the adjuster to the point where the canchain goes quietish and left it at that, I don't want to overtension. What's your approach?
28/08/2020 08:18:19 UTC
Bob said :-
Yes, it can be difficult to judge.
On the KLX it's really quite difficult because there is a second spring tensioner in the crankcases (no idea why). It's not a ratchet, just a spring but it means that as you back the manual tensioner off the camchain doesn't start to rattle - you can back it off quite some distance and still no rattle, then the chain jumps a tooth (been there, done that).
So on the KLX the only way is to set the stationary engine on the rock at TDC and then back the tensioner out and screw it back in until you can feel the resistance suddenly increase, then back it out 1/2 turn and lock it off.
I leave it alone until some miles later I can hear a little camchain slap, then I tighten it in 1/2 a turn and so on.
1/2 turn is 0.625 mm so I think that's a nice adjustment.
On "normal" bikes like the CBF what you are doing is what I would do.
28/08/2020 09:46:07 UTC
Ren - The Ed¹ said :-
Well you'll all be please to hear... IT LIVES!! It's not back on the road yet as I'm awaiting delivery of a fuel pipe, mine had gotten, erm, "baggy".

Looking around online Bob most folks seem to go with "tighten till the rattle stops then back of a 1/4 turn". Then of course there are the naysayers. This could be too loose and cause the cam to slop back and forth due to the valve spring load "pushing back" as the valve opens then "pushing forward" as the valve closes. I... erm I'll just tighten it enough to keep it quiet.
29/08/2020 11:40:58 UTC
Pocketpete said :-
Could it just be the bike telling you its had enough of this life. Poor thing.
31/08/2020 07:46:10 UTC
Ren - The Ed¹ said :-
Poor thing?!?!? It has just been lavished with a vast quantity of love and care AND AND AND some money has been spent too. I am of course now under sedation and awaiting counselling due to the trauma of having to open my wallet. If I don't get another 90,000 miles out of it I'm gunna be gutted.
01/09/2020 11:12:51 UTC
Bogger said :-
If that bike makes another 90,000 miles I'll buy you a new one myself. By the way, to achieve this, it cannot become another 'Triggers Broom'.

01/09/2020 12:50:01 UTC
Ren - The Ed¹ said :-
What are your conditions to achieve this wonderful offer then Bogger?
01/09/2020 12:55:14 UTC
Bogger said :-
I suppose the main one would be not connecting an electric drill to the end of the speedo cable. But knowing your luck, the cable would snap anyway. But like I said 'Triggers Broom'.

Can you not just blag a new bike off Honda for ' A real world, long term road test'?

You've got to start thinking outside the box a little.

01/09/2020 02:00:44 UTC
Ren - The Ed¹ said :-
Yeah, like Honda are likely to just hand over a nice shiny new 125 to ME!
01/09/2020 02:55:50 UTC

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