Strange Tach/Speedo

[724thomas]

I'm pretty sure I haven't heard this one before...1970 R75/5..Excellent recent rebuild (5,000 miles ago) includng a Palo Alto rebuild of the Instrument pod.
Today was perfect for an air cooled bike. 55 degrees & sunny, bike very happy to be invited to my ride...and suddenly the tach jumps up to almost double my actual RPM and the Speedometer jumps to double my speed. The bike itself continued to run great.
I pulled over and shut down the engine for a few minutes then re-started, and the symptons remained the same. I did hear a very muffled "whrrrr"ing sound from the front of the engine that remained constant with the engine rpm.
Four hours later at lunch I started the bike and the noise and the faulty readings were gone. At 5 o'clock I rode home with no hint of the dancing gauges.
Is there something I should be checking? or am I going to wait to see if it returns. Any hint of where to look???Thanks TomJ

[Garnet]

Haveing owned a small number of 70s airheads including 3 /5s I would say that the Motometer speedos and tachs are crap. How they ended up on such an expensive bike is beyond me. I have heard complaints about Palo Alto before, but I would not rush to blame them as it is hard to make a slik purse from a pigs ear.

Has anyone put any miles on the new aftermarket /5 speedos from Germany? Scroll down a bit on this page: http://www.ulismotorradladen.de/englisch/englisch.htm

They have been out for a few years now and are being sold by a few people on the net.

[SteveD]

I did hear a very muffled "whrrrr"ing sound from the front of the engine that remained constant with the engine rpm.

Is that where the cable comes from? Might be worth a look, and some lube?

[mattcfish]

The Tach and Speedo are seperate cables. To have them both act up at the same time....I can here a Rod Serling narative coming on. I think you may have encountered some sort of imbalance in time and space.

[Major Softie]

I think you may have encountered some sort of imbalance in time and space.

I hate it when that happens...

[724thomas]

I got a chance to open the headlight. The rear cable (Tachometer) had backed out of the housing....but the front cable (speedo) was in tact. I guess I'm okay now but I'm still baffled as to why both gauges would act up when one cable was loose.
I know that each gauge has to translate the rotary motion to the needles but I always assumed that was done by a independant spinning magnet. Maybe they use an electromagnet fed by a wire in series so one failure causes the other to fail. What other common connection could there be???TomJ

[Major Softie]

I do not believe there is any electrical connection in mechanical Speedos and Tachs except for Illumination and idiot lights.

[tomjohnston]

I know there is no wire connection, I'm wondering since a tach or speedo is basically a minature magneto, generating it's ownDC current with a magnet to move a needle, maybe there is some low voltage " going on....Hey! I'm just fishing for an explanation.
Now I have everything hooked up and my speedo is stable but my tach is still reading double the rpm's. I think somp'in busted.TJsomething"

[mattcfish]

I know there is no wire connection, I'm wondering since a tach or speedo is basically a minature magneto, generating it's ownDC current with a magnet to move a needle, maybe there is some low voltage " going on....Hey! I'm just fishing for an explanation.
Now I have everything hooked up and my speedo is stable but my tach is still reading double the rpm's. I think somp'in busted.TJsomething"

I don't know about the speedo, I don't recall a magnet, just slipping gears. A little epoxy on the spindles and I've fixed my speedo and odometer before.
I had a similar problem to yours with my tach on my /6. There is a magnet under the tach face that transfers the rotation of the tach gears to the needle. There needs to be a gap between the magnet and the drive mechanism. Nothing is supposed to actually touch the needle. On mine the gap had gotten too small and the drive mechanism would touch the needle pivot and move it to the redline and beyond. I can't remember what I did, but some how I tweeked something and got the gap back and the tach has worked fine for the last 10 plus years.

[tomjohnston]

Thanks for the clarification. I suppose the cable coming off the tach allowed the inner drive to jump around and do some damage.I'm still baffelled(sp?) why the two would fail at the exact same second. I wasn't horsing around...just happened at cruising speed.
Major Softie's "Time and Space imbalance" must be looked into a little closer.Thanks guys. TJ