Very Jumpy Speedometer

Hello all,
The speedometer on my 82 XJ550 has been having some issues lately. It is jumping all around and I cannot seem to get it to read the proper speed. Sometimes its way to fast, others its too slow. I tried to clean it out as best I can with no luck. One thing I noticed is the piece that has the magnetic metal disk attached to it (first gear to drive anything) has a lot of up and down play in it. I have the whole speedo apart and I am trying to diagnose why sometimes it runs faster and other times slower. I want to rule out it being gummed up because it hits both ends of the spectrum. One time jumping up so far it broke the needle.

Any ideas?
Thanks

 

Have you checked the cable? a failing cable can cause this . The Speedo drive is it seated in if front wheel has been removed recently ?

 

I checked the cable for any wear or tear and it looks great. Is there any other way to rule out the cable other than visual inspection? I have my drill hooked up to it which has a rpm of 2000, so I know what my speedo should be showing, and its definitely off.

 

Did you lubricate the cable as well, even if it spins?

 

Yep! Everything is well lubed. The cable appears to have almost no resistance. It seems to me it has something to do with the magnetic field generated by the spinning disk and its effect on the cup. I really wish I could take it apart further but since they make it tamperproof it won't be fun to take it apart any further than here.

 

https://www.xjbikes.com/forums/threads/speedo-and-tach-dissection-with-pics.15303/#post-126943

 

I've read that to get an idea of what I can and cannot remove without cutting the tabs holding everything in place. My next idea to fix this is to put something in place that does not allow that first drive gear with the magnet to move up and down. I'm convinced that is what is causing the issue as it would have an effect on the magnetic field. Anyone who has taken these apart..... did your first drive gear that holds the magnet move up and down?

 

Your speedo drive at the front wheel can need lubed , and might not be locked in place correctly there are 2 nipples on fork that drive is slotted in , take a look at this.

 

I just finished checking that and no luck there. Everything was properly in place. It’s either the speedo or the cable. I have it hooked up to my drill, and that turns at 2000 rpm. Well I can’t get a consistent speed out of the speedo. I was getting 49ish, and then for repeatability I unhooked the cable hooked it back on and only got 40. Then I did so again and got 65.
I’m so confused

 

Maybe it’s time to say goodbye to a speedo and go nascar style. Only have a tach.

 

If all else fails get another speedo .

 

So update as of tonight I took my speedo even further apart than in that post listed above. Using a dremel I got every single part apart. I think I know what the problem is. The magnetic disc has enough play that it can ride the cup and spin it super fast. So, I’ve JB welded everything back together after cleaning it all. And hopefully my JB weld repair on keeping the disc in the same spot works. There was actually wear inside the cup so it’s been doing so for a while. Will know the results tomorrow when the JB weld cures.