Bouncing Tachometer Needle

[McG99]

Hoping my video upload will work to show the tachometer needle bouncing. 
I've watched a few videos that say this may be caused by a loose wire(poor connection) or possibly a dirty cable, and or a grease that needs to be replaced.
I have a parts bike so I'm probably just going to swap the tachs soon and look into the problem later but wanted to see if any one here has had this issue and what the fix was, if the problem was found.
This is on a 1991 CB750

https://youtu.be/NzQhmc1rubY

[draser]

In Magna world most common cause for this is a "defective" spark module (where the tach signal comes from). I'd check integrity of connections at spark module and tack first, look for corrosion, loose wire, etc.

[McG99]

Thanks.  I've seen a few videos where they talk about loose connections. For now I'm going to swap out the tach hoping that the spare parts bike I have has a working tach so I can just swap it and not having a bouncing tach for now.  Then at some point pull the other apart and see what I can see and maybe fix it, as a back up then. 

[77tarheel]

This is a common issue with these era Nighthawks. I had a '93 750 with the problem. It's usually a cold solder joint on the clockspring in the tach. I repaired mine a couple years ago, but sold the bike earlier this year. Here is my write-up on how to do it: https://www.jgaryward.com/wordpress/wp-content/uploads/2021/07/2TachFix.pdf

[Pete in PA]

Here's hoping mine behaves.  Looks pretty involved to fix, get just right, and still look good.

[McG99]

Fortunately I had purchased a "spare parts" bike in 2019.  Swapped the tach off that one and installed on the riding bike and it's good to go.  I've kept the old tach and may some day try to fix it.

[aycaramba]

tarheel: This is an excellent write up--thank you so much for taking the time to document it and post it here.  I'm having this exact problem and will likely need to attempt this as a winter project.  Your write-up will be a fantastic resource as I work through this.

Yet another example of how grateful I am that this forum is back.

[77tarheel]

You're welcome. Take your time and be careful with the springs and you'll be OK.