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Home Repair And Restoration

CBF125 Speedometer Drive

Job Date 3 April 2020

By Ren Withnell

As I was returning from a job in Runcorn on Thursday 19th March, about 5 miles from home I'm travelling at an indicated 40mph. Then, as if by magic, I'm doing 20mph. I have not physically slowed, the engine noise is the same, I haven't changed gear and yet the speedo is showing 20mph.

Being an imaginative chap I momentarily pondered whether the universal laws of physics had changed, then the needle jumped up and down a bit. Nope, something has partially failed. 

Experience tells me it will be the speedo cable. After all this cable has, erm, 28, 29,000 miles on it (replaced at 61,000 miles). The cable will have snapped but the turning action will still be dragging around the cable further up after the break, hence the slowed speed. Dagnammit, I'll have to order one off Fleabay.

I put the bike away, little knowing this would be the last time it would get a "proper" ride out for some time. After this day I would be working from home. After the following Monday (23 March) we would be on Covid-19 Lockdown.

I'm sure it's the cable but I know better than to be sure, I need to be certain. I remove the cable at the clock and pull out the inner, the turny bit. Nope, that's fine. Hmmmm. I slide the inner back in and turn the end with my finger. I can feel some tension then clicking from the cable. Perhaps the worm end is worn?

The ends of this speedo cable are "squared". Some hefty machine will press the ends from round into a square shape. This square shape engages into a square hole at either end to allow the worm end to grab the cable and spin it, at the clocks to grab the cable and cause the mechanism to spin. I suspect the square hole at the worm end will have lost some of it's squareness. It is plastic after all.

The spiral wound cable is pressed into a square section at the ends of the speedo cable
At the ends the cable is pressed into a square section.
The small plastic worm has a square hole into which the square cable fits
The square hole in the drive mechanism that drives the cable.

I remove the worm drive and take a look. The square hole looks OK but as my eyes are not fitted with vernier calipers or laser micrometers what my eye tells me and what is true are 2 different things. 

However when playing with the worm I can turn the drive gear against the worm and get a clicking. Maybe the worm is worn? No matter, if it's the square hole or the worm the drive mechanism is one complete unit. I order a used replacement off Fleabay complete with a used cable for £20.

This arrives, I fit it and I take the 125 for a shakedown ride to the shops (essential journey M'lud), and all is well. Job done. Sorted.

The worm drive, speedo pickup, wheel mechanism, call it what you will, comes as a unit. The gear can but purchased separately but the worm and the housing are not designed to be disassembled. The housing is metal with the worm being plastic. The worm is held in place with a thick plastic oversize washer that is pressed into place.

The speedo drive unit in pieces with the housing, worm drive, gear and pressed washer
The disassembled speedometer drive.

How do I know this? I disassembled the mechanism. The thick oversize plastic washer was removed with brute force, rendering it useless but the unit was already useless. Now with the worm removed I can see the cause of my issue. Bear in mind this has 89,000 miles on it. Not bad for plastic.

The worm drive ought to be straight be we can see it has a deep curve worn through it
There's a little wear in the worm drive I think.

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Reader's Comments

Bill said :-
The worm has turned Ren or in your case it hasn't:-)
16/04/2020 09:00:28 UTC
Bogger said :-
If bits of plastic are wearing out at only 89k you really need to buy another bike.

16/04/2020 09:12:32 UTC
Ren - The Ed¹ said :-
Aaahhh Bill - I'm reminded of the Two Ronnies and their ongoing sketch of them living in a Matriarchal Society.

Another bike Bogger? I can barely get out the house to buy milk let alone another bike. Anyhow - who's going to pay for this 'ere new bike? It seems the sponsorship deals aren't forthcoming, my modelling career is going nowhere and despite my technical geee-nee-ooos in motorcycle development and aerodynamics I haven't been discovered. Life is cruel.
17/04/2020 08:46:24 UTC
Bogger said :-
Simple. Just ask Sharon to buy you one. Sorted. Or get one for free somewhere.It just needs a bit of lateral thinking.

17/04/2020 09:27:38 UTC
Sharon said :-
I am afraid solution 1 - Just ask Sharon to buy one is a no goer
17/04/2020 09:41:13 UTC
nab301 said :-
Obviously the drive is worn, most likely mileage related, but is lubrication of said gears on the maintenance list?
17/04/2020 07:27:25 UTC
Pocketpete said :-
Is it a case of round peg in a square hole.

Shocking lack of maintenance if you ask me.
17/04/2020 08:08:51 UTC
Ren - The Ed¹ said :-
So that'll be a no from Sharon then!

nab301 - whenever the front wheel is removed I clean and repack the speedo unit with fresh grease. So not "regular" lubrication but it is done when the wheel is removed.

Yes Pocketpete, a shocking, appalling and distressing example of terrible maintenance. I hang my head in shame.
18/04/2020 08:57:48 UTC
CrazyFrog said :-
My last brush with a Japanese bike ended badly (as usual), this time the straw that broke the camels back was the speedo drive.

The bike in question was a KlE500, you know, the trail bike version of the ER500 which is meant to be indestructible. 1st off, I found that the the alloy on the rear wheel was rotten, and there were huge cracks round the spoke holes. I was not impressed. Yes, the bike was 15 years old, but it was very low mileage. I acquired a 2nd hand wheel for just shy of £100. A couple of weeks later, and the speedo suddenly stopped working. Ah, I thought, the cable has snapped. I get a new cable off eBay, but on going to fit it found that the old one was OK. Further investigation revealed a genius piece of design. The brass gears in the speedo drive box had seized, and then the seized drive box ripped the end off the front wheel hub. Crap engineering and crap metallurgy! That'll probably be my last experience with a Japanese bike as they have been nothing but trouble for me over the years. :-(
18/04/2020 05:02:35 UTC
Ren - The Ed¹ said :-
CrazyFrog!! But but but... the KLE500 IS indestructible!! Everyone knows that surely. Aaaahhh... nooooo... wait, sorry my mistake. It's the KLR650 that's indestructible it seems you purchased the wrong bike. Silly boy.

OK the rear wheel is a bummer. And the speedo. But as you'd replaced the rear wheel why not just fit a bicycle speedo to it? My CLR125 did the same, ate the worm drive mechanism. I fitted a magnet to the front wheel with araldite, fitted the pickup to the fork leg and fitted the digital speedo to the bars. £10 and once it had been programmed with the wheel dimensions I was good to go. The only reason I haven't done the same with the CBF125 is I'd like to see the mileage climb some more. If we ever get out of LockDown that is.
19/04/2020 05:11:26 UTC
Ian Soady said :-
A pushbike speedo was good enough for my Sunbeam.
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19/04/2020 05:17:54 UTC
Ren - The Ed¹ said :-
Illumination Ian? I'm sure a gentleman of your demeanour doesn't ride at night but I had to fit an LED bulb to a bracket over cycle speedo for night use. My MOT inspector raised an eyebrow but was more than satisfied as to it's function.

Many friends were concerned that it would not register high enough speeds. Indeed the speedo only went up to 99mph (2 digits) but being fitted to a 125 AND being almost 30mph over any legal limit in the UK it was more than sufficient. If I'd been in KMH though it could have been an issue.
19/04/2020 05:40:36 UTC
CrazyFrog said :-
I did consider the bicycle speedo option Ren, but wasn't at all sure it would pass an MOT. In the end I managed to get a replacement front wheel for about £150. Interestingly, the vast majority of 2nd hand front wheels on eBay exhibited the exact same damage to the front hub caused by a seized speedo drive. Anyway, I fitted a new front wheel and moved it on in disgust...
19/04/2020 08:53:44 UTC
Ren - The Ed¹ said :-
I guess you just ain't had luck with Jap bikes.
20/04/2020 10:45:59 UTC
Ian Soady said :-
I thought you quite liked the SLR650 Pete apart from its tank size.

WRT lights on the speedo I seem to remember that some come with an internal light. The MoT manual doesn't mention speedometers (see link) for bikes although it does for cars.

Oh, and I didn't need one on the Sunbeam anyway as it was pre 1937 (I think that was the cut off date). Speedos were an optional extra - as indeed was lighting!
20/04/2020 10:59:50 UTC
CrazyFrog said :-
Yes, the SLR650 was one of the better ones Ian. The tank size was the big issue for me, though I also had the starter motor fail, which nearly stranded me. I managed to get a rebuild kit for the starter motor fortunately...
20/04/2020 03:10:29 UTC
Ren - The Ed¹ said :-
Yes the tank size on my SLR650 was a source of frustration. After 90 miles it's time to start thinking about filling up, 100 miles is fine but you really need to stop soon, 110 is getting sweaty. I was very close to purchasing a CRF250L but again the tiny tank would be just so frustrating.

So CrazyFrog if it ain't from the land of the rising sun, which country supplies bikes you can live with?

22/04/2020 07:02:25 UTC
Ian Soady said :-
"So CrazyFrog if it ain't from the land of the rising sun, which country supplies bikes you can live with?"

I can tell you you won't like the answer.....
22/04/2020 10:13:38 UTC
Ren - The Ed¹ said :-
Aaaah yes, it's all coming back to me now. Something to do with Eastern Europe, noisy smelly engines and enclosed chain cases. I feel queezy...
22/04/2020 12:15:56 UTC
Bob said :-
Crazyfrog - I had exactly the same with an FZR600R, brass gears and a lack of lubrication (before I owned it).
Maybe that's why they use plastic now, so if it seizes it just destroys itself and not the wheel hub.

I've had 4 or 5 KLE500s and I found them to be utterly indestructable, even when I cartwheeled one into a ditch at 60MPH following a front tyre blowout - I dragged the bike out, it started up and I rode it two miles home on a shredded tyre.
I was working on one once and accidentally connected the carb heater line (full of coolant) to the vacuum takeoff on the left hand inlet stub. I could hear the noise of running water. The bike wouldn't start. I eventually realised, drained the left carb using the floatbowl screw, took the left plug out and cranked it over to blow all the water out of the cylinder, refilled the coolant and it started and ran a treat and caused no problems.
The bike was built like a tank.
There's not much Kawasaki can do about a lack of cleaning, the abundence of road salt and the passage of time.....

The only reason I don't still run one is weight. (the bike's weight not mine).
22/04/2020 03:06:50 UTC
nab301 said :-
@ Ren , the ultimate thread hijack , get the Rennies out now before you start feeling Queasy...
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22/04/2020 08:32:37 UTC
Ren - The Ed¹ said :-
Urgh. Gip. Shivver. I just brought a little bile up into the back of my mouth.

I watch a lot of youtube vids and the Yanks are always swapping out this motor for that, squeezing a 7 litre
Cummins diesel into a Ford pickup, you get the idea. There is in fact little wrong with the MZs save for that nasty noisy smelly engine. Maybe I should buy one and swap the motor out for a Yanmar Diesel.
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23/04/2020 08:36:54 UTC
Bob said :-
That's harsh!
I had great success with and still have an abiding admiration for the MZ ETZ250.
Most unlike a japanese two stroke they are all about torque and long legged fuel efficient lolloping along.
Then I had a daliance with Jawa 350 twins, these even more so, what a glorious (but very smelly) device.
23/04/2020 09:27:32 UTC
Ren - The Ed¹ said :-
I will never get my head around 2 stroke. Like right, OK, so we let air/fuel into the bottom of the engine where there really ought to be oil. Oh - I know let's stick the oil in with the fuel! Yay... Then when the piston comes down full of flame and burned fuel (and oil) we'll open the exhaust port at almost the same time as we open the inlet port. We'll sort of hope for the best that exhaust gasses go out the exhaust port and fuel/air/oil gases will come in through the inlet port. You see the piston coming down will pressurise the crankcase... a bit... perhaps... No, no, they won't get mixed up and go the wrong way, trust me!

2 strokes work on the principles of hope and probabilities. Very highly tuned 4 strokes do as well by having overlap between the exhaust valve closing and the inlet port opening. It's just WRONG!

I can tolerate, accept and even appreciate the massive 2 stroke diesels. These at least have a properly oiled crankcase with properly oiled conrods and wrist pins, properly pressurised forced induction to ensure the gasses flow in the right direction and a bona fide exhaust valve.

In my perfect world I'd like a 400cc sidevalve diesel with mechanical injection. Probably weigh almost a tonne, achieve 15bhp and a max speed of 45mph, but by god it'd be easy to look after.
23/04/2020 10:14:29 UTC
Bob said :-
Hah hah, I know what you mean.
The wierd world of fluid dynamics - it shouldn't work but it does.
Walter Kaaden was your man, MZ invented the expansion chamber which is pretty much why they won every race going until their star rider defected and took the secrets to Japan.
Interestingly the other day I saw an artical about a supercharged direct injection 2 stroke with a sort of flapper valve in the exhaust port. The advantage being it has a 4 stroke bottom end.
I'd thought about this before. You could relatively easily (he says) put a flat 2 stroke head on a 4 stroke engine, stick some ports in the barrel and put a blower on to force the incoming mixture.

Having last year built a 1KW electric bicycle I am quite assured that the future is actually electric, soon there will be batteries that are useful in terms of capacity and recharge time and we'll be set.
23/04/2020 11:17:09 UTC
Ren - The Ed¹ said :-
I agree regards the leccy bikes. If/when some boffin cracks either/both the charging time and/or the capacity/weight issues I doubt there'll be any more combustion engines. These solid state batteries look promising but we're not there yet.

I do like a good combustion engine, they have character and lots of oily bits. It is staggeringly amazing how reliable modern fuel engines have become considering their complexity. But then with the limited number of moving parts in an electric motor there's not much to go wrong. Of course there'll be a computer to throw a hissy fit just to keep us on our toes.

There'll be similar cycle parts too that'll break. Head bearings, wheel bearings, suspension and tyres will likely be the same.
23/04/2020 11:37:11 UTC
Bob said :-
Electric bikes aren't soul-less though.
Mine like most is torque heavy but power light, so there is that lovely shove in the back that riders of big singles and V twins know so well.

There is a computer but it's less complex than the one running a petrol engine and crucially it doesn't care about ambient air pressure or temperature, there's no O2 sensor, no coolant so no coolant temperature sensor, no knock sensor, no crank position sensor. Just a TPS in the twistgrip and hall effect sensors in the motor itself. Mine has literally just one moving part, the wheel itself which is suspended on 6202 sealed ball bearings (the same as in the KLX250 front wheel).

If and when I can buy an electric bike that'll do 70 MPH on the motorway for 100 miles then recharge in minutes (and we're not far away now) I for one wont look back.
23/04/2020 01:46:06 UTC
nab301 said :-
Bimota with their V Due direct injection 2 stroke were the first but I guess it nearly sank them... What's not to like about two stroke , Lightweight often aircooled engines ideal for commuting , with modern synthetic oils there is no visible smoke , you can even run them in the ULEZ zone .
I put 50k miles on my MZ with no internal engine components fitted.
Diesel engines conversions are available in the UK but a Yanmar or Hatz diesel is really a stationary fixed speed engine so absolutely no powerband =no fun whatsoever
There's even MZ's with diesel conversions...
23/04/2020 02:13:55 UTC
Ren - The Ed¹ said :-
I hope you're right regarding batteries Bob but I don't as yet feel as though we're as close as I sense you think we are. Shame, because I agree they might not go "brum brum" but otherwise there's no reason to think they won't be any fun. I think the instant torque will be rather addictive.

I take your points on the 2-stroke nab301 BUT BUT BUT, oh I give up. Yes the Yanmar type motors are designed to drive farm machinery (augers, mulchers etc) and generators and would be about as thrilling as this lockdown is proving to be. However it would get you there. Eventually.
23/04/2020 02:28:44 UTC
Bob said :-
I saw what looked like a successful conversion with a V twin diesel driving through a snowmobile CVT
23/04/2020 03:48:38 UTC
Borsuk said :-
I have a 7 cylinder inline two stroke engine you can have to play with if you want Ren.

Free to a good home, you can pick it up anytime. Figuratively that is.

It's a diesel, lots of very low end torque.
23/04/2020 10:57:07 UTC
Ren - The Ed¹ said :-
Borsuk - I am guessing you are talking about one of the massive marine engines on your dingy? As previously stated I can live with the diesel 2 stroke system as a) the crank, conrods and wristpins are properly lubricated, b) the forced air induction ensures the gasses have at least some chance of going in the correct direction.

How big is your free diesel stroker? Do you think it'll fit in my CBF125?

While I am sure Borsuk knows how the diesel 2 strokes work, for the uninformed see the video below.
24/04/2020 08:06:40 UTC
Borsuk said :-
Not sure of the CC, actually no idea what so ever, she does put out 24,000 Bhp at around 100 rpm. Weights roughly 500 tonnes and burns 140 or so tonnes a day at slow speed, so you might need to fit a slightly bigger fuel tank, ours hold about 7,000,000 litres.
24/04/2020 01:38:25 UTC
Bob said :-
Dunno Ren, that 2 stroke diesel has the inlet ports open at the same time as the exhaust port.
Are you sure you wouldn't prefer a steam engine with sliding block valvegear?
24/04/2020 04:01:52 UTC
Ren - The Ed¹ said :-
24,000 bhp you say? 500 tonnes? 7 million litres? I think the old CBF125 is gunna need a bit of work to make that happen. Wonder if the tyres will handle the weight?

Bob - it's the turbo/supercharger that helps keep things going in the right direction - however - sliding block valve gear, ooooooohhh yes now you ARE talking my kind of language. A steam powered bike has been done before but you got me thinking. I reckon I'd get one of them little Manod steam engines in the 125. Not sure about the startup times though, not exactly a turnkey solution.
24/04/2020 06:04:30 UTC

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