Cb77 speedometer and tach giving false reading

[Frank]

After disassembling, cleaning and a lube soak with marvel mystery oil on a speedometer and tachometer assembly on my Super Hawk project, they both read much higher than they actually are. Does anyone know what is wrong and how to fix? Thanks Tootie

[Geoff Hastings]

This is just a guess but, I suspect the oil is too viscous, I'd try washing the units with a solvent to remove the lubricant.

[DJM]

I'm sure Geoff is right, soaking them in oil was a mistake.

There should be just enough oil in there to lubricate the bearings and no more.

The needles are pulled round by magnetic attraction across a tightly controlled AIR gap, filling this gap with oil isn't a good idea. If you flush out the excess oil with solvent you might be lucky and get them to read accurately again.

Good Luck.

[Tim Allman]

I had a similar problem once and it resulted from the presence of oil where it shouldn't be in the two instruments.

For each instrument there is a small (~2 cm) cup which is driven by the cable. It is held in place by a bearing of porous, oil soaked brass or bronze. Above and inside it is a smaller drum with a layer of magnetic material on its side. When operating, the cup rotates and the magnetic layer is dragged along lightly by the formation of eddy currents in the cup. (This is the same effect that generates current in the alternator.) This force is proportional to the speed of the cup. The drum, to which the needle is attached, rotates by an amount proportional to the speed of the cup (and therefore proportional to the motorcycle speed or the engine speed) and we read off the speed on the dial.

If there is oil between the cup and the drum, there will be much more drag than there should be and the needles will read much higher than they should. Any oil here should be cleaned out but a general solvent wash will also de-lubricate the bearings. After cleaning, I used a small syringe and needle to put a very small amount of light oil on the bearings and all was and remains well.

[Tim Allman]

I'm sure Geoff is right, soaking them in oil was a mistake.

There should be just enough oil in there to lubricate the bearings and no more.

The needles are pulled round by magnetic attraction across a tightly controlled AIR gap, filling this gap with oil isn't a good idea. If you flush out the excess oil with solvent you might be lucky and get them to read accurately again.

Good Luck.

Sorry for stomping on your answer. It looks like we were writing at the same time.

[Frank]

Thanks guys! That's exactly what it was. I understand how it works now. I know better to do that again! I flushed out the drums with solvent and tried it out with a drill and it seemed to be normal again. Now, Where EXACTLY do I lube? Thanks again!

[brewsky]

FWIW...here is how I calibrated my Dream speedo after cleaning and lube:

1. Ride bike at constant speed (I used 50MPH) and check actual speed vs GPS or other known correctly calibrated speedo riding with you.
(I used a hand held GPS zip tied to the steering damper

2. Note the difference. (When my speedo said 40MPH, my GPS indicated 50 MPH actual)..10 MPH off!

3. Dissamble the speedo.

4. Connect the bottom end of the speedo cable to a reversable drill, run the drill wide open (in reverse) and note the indicated speed on the speedo. (my speedo indicated 38 MPH with the drill running wide open)

5. Rotate the tab on the needle spring stop to either increase or decrease the tension on the spring.
(since mine was reading low, I needed to decrease the spring tension)

6. Re-run the drill and see if the new max reading on the speedo is increased (or decreased) by the same amount as the original error. (My error was 10 MPH, so the new drill max reading should be 48MPH on the speedo instead of 38MPH.) This is a trial and error process.

7. Repeat 5 and 6 if necessary.

8. Reassemble and reinstall the speedo.

When I rechecked the bike on the road against the GPS, it was dead on the money.

Note: No longer need to have a GPS unit, as you can now download GPS apps to cell phone