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Discussion Starter · #1 · (Edited)
A while back the speedometer on my '73 T140V stopped working, and it turned out that the cable had gotten crimped severely. Picked up a new cable and installed it, but the speedo still doesn't work. It looks like the reason lies in the drive gearbox. I detached the cable from the drive and connected it to a drill, and the cable/speedo work fine. I sprayed some WD-40 inside to hopefully clean out any gunk, and greased the gearbox at the nipple on the bottom. Still nothing. Is it possible that when the old cable was crimped it burned out the gears because the cable wouldn't turn?

Along the same lines; I also replaced the tach cable at the same time because the outer casing on it had cracked. The needle bounces around a lot (it did with the old cable, too). I checked the tach, and using the drill the gauge runs smooth--no bouncing. I opened up the tach drive and cleaned out the gears, but it still bounces while riding.

The bike is pretty much my daily rider, so the speedometer is pretty important. Any suggestions for either problem? Am I overlooking something?

Thanks,
Joseph
 

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suggestion

leave it on the bike and take it off the speedo.

fire up the bike with the tire blocked up free of the ground (make sure it can't touch while it's running or you and the bike will sail into your beer bottle recycling bin)

fire it up, than use your fingers to test the spin on the cable. pull it in and out and see if it grabs intermittently.

the drive square on the engine gets reamed out occasionally for a number of reasons, usually a short cable or not getting a good turn down on the cable connector nut which lets the drive pinion (?) ream itself out with the cable. you should be able to stop the cable with your fingers if the drive guts are shot. figer pressure is more than enough don't try it with pliers that turns the inside into a whipping snake if you clamp down too hard
 

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Speedo

The speedo gearbox works on a worm and wheel system. As your bike is 36 years old, it is possible that the worm is worn out (usually the worm and not the wheel, I don't know why). You can remove this to check it by pulling off the end cap on the gearbox (the opposite end to the cable entry). Grip it with some mole grips and then twat those with a hammer to pull it out. You then fish the worm out with some needle nose pliers.
If that has happened it is stuffed.

Alternatively, the square end of the worm where the cable fits can round off. If that has happened, generally you are stuffed, but I have had a little success by pushing a piece of plastic tube into this end and pushing the cable into that. Works for a while, you just have to renew the plastic tube every now and again.

Your speedo is working. Good, that may well fail soon too....We are all getting old...Did I tell you about my bad back???? And I need to pee a lot....Hey, good reason to get your crank dynamically balanced, cuts out the vibration on your bladder...

Anyway, if you have a drum front end (you should have I think, front disk came in, in 74), then why not use a bicycle speedometer. Fit the magnetic trigger to a spoke, measure the wheel o.d. and programme it into the little speedo. Much more accurate and for only about £5.

If your gearbox is stuffed, rather than pay lots of cash for a new smiths, if you are any good with a drill, you can buy one of the Indian speedos that are made for Royal Enfields. The guts fit into the smiths speedo with some re-jigging of the rivet holes. Fraction of the price!

Mark
 

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I have first hand experience with this. The speedo drive on the rear wheel is gone. When the cable siezes, it causes the driven gear to wind up the cable like a spring between it and the seizure until it cannot wind it up anymore. Then the gearteeth start to skip off the drive worm and the wound up spring causes it to probably jump a few more teeth. Then the process repeats again until a nice radius, the same size as the OD on the drive worm is formed in the driven gear.

The cable does not have to completely sieze in order to do this either. I can be rusted inside the cable housing and just turning hard. At high speed it will start to put enough pressure on the gears to force them apart with the same subsequent result.

That's why it is a good idea, especially for anyone with an original Smiths gearbox on their bike to remove the cable, clean the housing and cable and then relube the cable. It's an easy job and worth doing at the end of every season.

I've taken a worn out Smith's gearbox apart. It probably could be rebuilt with the parts from a repro gearbox, but unless you have a 100 point bike, I don't think it's worth it. If I'm not mistaken the driven gear assembly is crimped permanently into the housing, so my thought on being able to potentially rebuild it may not be accurate.

Does anybody know if any of the speedo repair guys will rebuild the gearbox?
regards,
Rob
 

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good question

"Does anybody know if any of the speedo repair guys will rebuild the gearbox?"

i'll ask my instrument repair guy when he shows up to get his MGB cherried out. he repairs smiths. my guess is that the hard part is knowing where to find the parts.
 

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Discussion Starter · #6 ·
Thanks for your input. I appreciate all of it.

Bottom line is the speedo drive is toast, and I'm not up for dropping a few hundred $$ for replace it right now. I do have a disc brake up front (introduced on the T140 in 73), but I'm looking into Mark's suggestion of a bicycle speedo. There are a couple inexpensive, wireless versions that might work as an interim fix while I save up some cash for the real deal. Anybody have any other ideas?

How about suggestions for the bouncing Tach needle?

Cheers from wet and wonderful Seattle!
 

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First, I'm sure you can find them cheaper, but Klempfs lists the speedo gear drive for $79.00. Cannot remember what I paid for mine.

Your bouncing tach needle is probably a poorly lubricated cable. pull the entire cable off and clean it. I would soak the housing in mineral spirits and run a swab thru it on a piece of wire. You might be able to get a .22 cal bore brush in there. Cannot remember how big the ID is. Get it as clean as you can. Clean the drive cable as well. You might find it has broken strands that are catching on the housing. If so, get a new one. You should be able to buy just the drive cable. But make sure your housing is good. After cleaning put the drive cable back inside DRY and spin it with your fingers. See if it spins freely. Curve the housing the same as if it were mounted to the tach and engine and spin it to make sure there is nothing binding it inside. If all is well, lube it with a good light grease and put it back together.

I was going to put a bicycle speedo on my racebike just for grins. Never did, but have the speedo. Not many bike speedos go beyond 60 mph. The one I have does. I'll have to see what I paid for it if you are interested. You still have to rig up the sensor in order for it to work.

Since you mentioned your front brake, if you have not already, I would strongly suggest you go buy a can or two of new brake fluid and flush it thru that brake until it comes out nice and clean. You should change your brake fluid every 2 to 3 years on a street bike. Not doing so invites corrosion.
regards,
Rob
 

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Discussion Starter · #8 ·
Thanks Rob. It's a brand new tach cable, well lubed before I put it on, so that should be good. I'll check the drive cable and housing, though.

I'm currently rebuilding the front brake caliper and replacing the master cylinder and brake hoses, so the brakes should be golden by Monday. :)

Cheers!
Joseph
 

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First, sorry for not reading from the beginning before answering the tach question. Hard to keep all the content of these threads in one's head. The internal mechanisms for both the tach and the speedo are a spinning magnet that does not actually touch the mating component, and that component moves the needle. At least it is not supposed to touch. The faster the magnet spins, the more it moves the needle. The instrument is calibrated by changing the strength of the magnet. When you ran it with a drill, and it was smooth, that would indicate that the bushings are not worn and the lube is still doing it's job. But I would also guess that the 30+ year old lube is getting tired. That's another subject.

So, if you are sure that your cable is not binding and jumping, and we know that the tach itself works fine, then my guess would be that the drive on the engine needs attention. Yeah, I know, it's the only part left. Not rocket science. I have no experience with the engine drive so will leave that for comment by others.

If the the bushings are worn in the tach, I can see the possibility for the magnet to wobble and have that cause tach bounce because the distance (air gap) between the two components varies. It could also be chattering in the bearing due to wear or due to dried out lube. But again, your drill test would indicate that is not the issue here.

Make sure the drive cable inside the housing is not too long. It could be binding when you lock everything down. The one end buts up against the bottom of the hole in the little screwdriver type shaft for the gear drive and the other end buts against the tach hub. So if the cable is too long the cable takes the shape of multiple letter S inside and it flops around instead of smoothly spinning. Pull the housing off the tach and make sure the shoulder on the flange of the drive cable is slightly below the flange on the housing that the knurled nut locks against. Mine is about a 1/16" below the housing flange. If the drive end flange is above the housing flange, you could have a drive cable that is too long or their is crud in the gear end drive shaft hole. I also checked the engine drive end on my '76 and the screwdriver shaft sticks out of the housing just a hair over 3/16" from the end of the hex nut on the housing.

You could carefully chuck the gear end drive (screwdriver type drive) of the cable in your drill and see if the tach bounces. If it is smooth, AND you have proven to yourself that the cable is not too long, then either something is going on with the gear drive in the engine or it is just engine vibes making it bounce.
Keep us posted on what you find.
regard,
Rob
 
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