SOHC Speedometer Drive Rebuild

[Deer Slayer]

I'm in the process of rebuilding the speedometer drive on my CB750K2. I've been getting the bouncing speedometer needle blues above 40mph. I checked the cable, and it appears to be in good condition although the liner may be worn. It was time to replace the front wheel bearings so I thought I'd give the speedometer drive a good once over while I had the front hub apart. The gear teeth are in good condition and there is minimal thrust movement on the ring gear. I did check the worm gear thrust and found it to be 0.033", that seems a little too much to me. I was think of adding a bellville washer on the worm shaft to keep it from thrusting from vibration and bumps, as I now suspect the play may be allowing an oscillation that causes the bounce in the speedometer needle. I'm just looking for some feedback on the bellville washer idea. Has anyone tried this repair, if so how were the results? If you think I should just reshim to reduce the play, do you know how much thrust in the worm shaft is acceptable?

[TOOLS1]

I have never dealt with the drive gear, but the speedometer is magnetically operated, and sometimes a good cleaning will bring them back to smooth operation.
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[Deer Slayer]

I think I may try spinning the speedometer up by attaching an electric drill to the cable. It should tell me if the cable/speedometer are the culprits. I found a bad cable that way a few years back and replaced it. It was running smooth with the drill and new cable. I still had the bounce in the speedometer above 40mph. I've just been living with it until now. I thought it might be wind leaking into the speedometer case at speed. I put a windjammer fairing on about three weekends back and went for a test drive, that didn't seem to change anything. I'm pretty sure it's something in that speedometer drive.

[Deer Slayer]

I picked up a small wavy washer at the hardware store and got the drive reassembled. I left about .002" thrust in the worm shaft. It made an improvement, but still have some bounce above 40mph. I guess it's time for a new cable. If the cable doesn't do the trick, I'll tackle cleaning the speedometer magnet. Thanks for the advice.

[Deer Slayer]

The new cable didn't get the bounce fixed. Does anyone have a suggestion on what cleaning agents will work on the speedometer magnets without causing any damage to other components? I'm thinking alcohol would be safe.

[Deer Slayer]

I finally open the speedometer and cleaned everything with rubbing alcohol. I added a little white grease to the odometer gears. It seems that the damper is seized, and that's what is causing the bounce. After doing some research I found out they could be cleaned and refilled with 30,000 cst silicone oil as used in RC car differentiels. All the write-ups I found have photos of the damper with a lid that can be removed to refill it. Mine is of another design, like the one in the attached photo. Anybody have any experience refilling this type?

[Deer Slayer]

Finally solved the problem. I partially disassembled the speedometer and cleaned/oiled all the necessary points and reassembled and still had the bouncing speedometer indicator problem. It turns out that the bottom on the needle shaft sits in a guide located underneath the aluminum bell. That point needs to be lubricated with something thicker than oil or it will allow the needle shaft to vibrate. I used just a dab of automotive dielectric grease and finally got rid of most all of the bouncing. I chose dielectric grease because it is silicone based and has a very good temperature stability. I didn't want to put anything in there that would melt and drip onto the magnet in the hot weather.

[TOOLS1]

That's cool. Thanks for updating us on this.
TOOLS