Wobbly Jota Speedo

A

Alexb83

Full member
Location
Northumberland
Hello all, attempting to diagnose and fix a bouncy Speedo - any assistance much appreciated!

Withdrew Speedo cable from drive and clock ran steady indications on the end of a power drill. Found cable fixing nut packed out with 2x fiber washers. Is this normal?

When first loosened the drive housing came free separate from the worm gear which I then lightly pulled free.

Removed Speedo drive and gear. Both well lubed and no obvious signs of wear. Ran gear on drill with cable, again running steady on the clock which indicates the square cable end is snug in the worm gear.

Cleaned, regreased and going for reassembly... is there supposed to be a metal thrust washer at the base of the worm gear where it mates into the gear assembly? The fiche seems to indicate one placed at the end but none have come out when the worm gear was liberated. And how snug should the worm gear be into the assembly? If I insert the gear first and seat it solidly i then have fouling issues with the disc trying to run the drive housing back onto the spindle of the worm gear in order to close it all up.
I have more joy reinstating the whole thing as one but then can't be confident the worm gear has slotted in properly?

Thanks
Alex
 
Quick potential fix: new speedo cable
Likely not the root cause but may calm it down a bit.

The main issue is there is silicone damping fluid inside the speedo mechanism and this leaks/deteriorates with age.
Only guaranteed solution is to have the clocks professionally stripped and repaired.

I use Marcel's services at CB750Faces.com
 
Hi Grant - there's no speedo wobble at all when I run either the cable or the cable+drive together on a power drill. The clock indicates steady speeds at steady revs. Wouldn't that indicate the problem is at the bottom end of the assembly?



 
Could be fit of the squared cable into the drive quadrant then. If it's rounded off try a new cable.
 
I've had the same problem on my 3C.  Speedo ran steady when fitted to a drill, but fluctuated when fitted to the wheel drive.  Tried a new cable, which made no difference.   

What I found was that the inner cable that fitted into the wheel was not engaging securely, and that the square end of the inner cable seemed too short.  I had another inner cable made up with a longer square section.  This helped a lot, although still not perfect.  The cable routing also seems to affect the operation.

Even though it's still the original speedo (done approx. 120,000 kms) I'm also inclined to think that the issue is on the wheel drive side, rather than the speedo.

Tom
 
Its definitely at the bottom end, I've stripped, Cleaned, greased and rebuilt it a few times today. Each time the drive ends up either fully or intermittently seizing when the wheel is turned forward. Intermittent seizing would explain the bouncy Speedo.

And now unfortunately it's a wheel off job to investigate further.

On the last attempt I lost the bottom thrust washer inside the Speedo drive cover  >:( Its probably stuck in a dollop of grease but I don't fancy just leaving it rattling around the Speedo gears.

Hoping that the gears don't turn out to be worn beyond repair - at least there isn't any visible damage to either.

 
It's actually not a worm gear, but a crown gear. *smartass mode off*
If they get stuck and there is no apparent wear on the gears, usually the bearings are worn out and they start to "climb". Weakest link IMHO is the alloy housing of the drive, respectively the bearing seats of the pinion shaft.

Regards,
Stefan
 
The suggestions of the drive are all valid but as a start I'd remove the inner cable, clean it and the sheath with some solvent, lube with some light oil and see what happens. The cable spinning in the sheath with iffy lubrication will create a gummy metallic paste at the points of contact[bends] and cause the cable to wind up and spring when the torque overcomes the resistance. This has worked on about 90% of the old bikes and [mostly old british] cars I've worked on, even Hondas when they still used cables.
 
My 3C speedo wouldn't go above 60MPH it would bounce around between 57-63 or so, but never anything faster. I pulled the front wheel for other work/maintenance and had a look at the drive to no avail. Next ride it was working fine. So thinking the gear was skipping at that speed and reassembled in a way that the tolerances stacked up to my favor.
 
wdietz186 said:
The suggestions of the drive are all valid but as a start I'd remove the inner cable, clean it and the sheath with some solvent, lube with some light oil and see what happens. The cable spinning in the sheath with iffy lubrication will create a gummy metallic paste at the points of contact[bends] and cause the cable to wind up and spring when the torque overcomes the resistance. This has worked on about 90% of the old bikes and [mostly old british] cars I've worked on, even Hondas when they still used cables.
Click to expand...

My point exactly. Might be a cheap work-around just by cleaning the drive assembly and fitting a new cable. Has worked for me before to calm down an erratic speedo needle.
 
Well, I got the front wheel off and finally diagnosed the problem:-

1x crushed thrust washer in the speedo drive case.
1x cracked speedo drive case which seems to have occurred due to the broken washer.

The bouncing is probably down to the whole thing being slightly out-of-line with the crack at the bottom end where the gear seats itself.

I don't suppose anyone's got a spare case?

 

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You, sir - are a gentleman and a scholar :D

Later in the year I look for a weld repair on the old one, if possible. It hasn't massively deformed and it would seem like a shame just to bin it.
 
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