R75/7 Rev Counter Noisy

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Dave Stapleton

R75/7 Rev Counter Noisy

Postby Dave Stapleton » Sun Sep 17, 2017 10:08 am

I have an issue with the mechanical rev counter on my 1977 R75/7. When the engine is revved, whether at a standstill in neutral or accelerating through the gears, there is a loud clancking/rattling sound coming from the rev counter which can be heard above the engine noise. I have noticed that the rev counter needle is slightly erratic as well. Just to complicate things, if you raise the revs very slowly, or accelerate very slowly, the noise isn't so bad, or almost non-existent. Any ideas? :-k

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Galactic Greyhound
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Re: R75/7 Rev Counter Noisy

Postby Galactic Greyhound » Sun Sep 17, 2017 11:38 am

Is the noise due to a rusted inner tachometer cable?

Pull out the inner cable from the outer, clean it using WD40 and a cloth and give it a light greasing but leave the top 150 mm (6") at the tacho end clear of grease to avoid contamination of the instrument.

The inner cable is likely pulled out from the 'drive' end of the cable.

A sticking inner cable will 'whip' and cause all sorts of problems before eventually breaking.
Ced.

R1100RT 1996.
Sent by Boson Quantum Transmission from the Starship 'Galahad'.
http://www.researchgate.net/publication ... ing_bosons" onclick="window.open(this.href);return false; - It works!

Dave Stapleton

Re: R75/7 Rev Counter Noisy

Postby Dave Stapleton » Sun Sep 17, 2017 11:40 am

No, the noise is coming from inside the rev counter housing, I have checked the cable and that is fine.

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Galactic Greyhound
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Re: R75/7 Rev Counter Noisy

Postby Galactic Greyhound » Sun Sep 17, 2017 12:08 pm

OK - I haven't taken a mechanical tachometer apart but think that it likely works in the same manner as a speedometer i.e. the cable drives a rotating magnetic disc which induces Eddy currents into an aluminium disc attached to the tachometer needle which has a circular spring to keep it on zero.
The faster the magnetic disc turns the stronger the Eddy currents induced into the aluminium disc and the further round the needle rotates.

There is therefore no mechanical connection between the magnetic disc and the needle.
Any mechanical contact between the two discs due to a fault or debris would likely result in the needle going full scale deflection.

Any noise/vibration must therefore be in the rotation of the magnetic disc.

This could be due to debris between the magnetic disc and the housing or bearing failure.

I think that you need to remove the tacho from its housing for inspection.
It shouldn't be too complicated finding the cause of the problem.
First check there is no debris in the housing and then check that the magnetic disc spins freely on its bearing and is not loose.
Then check the tacho drive gearing to the disc is OK.

The above assumes it is a magnetic disc drive to the tacho needle which I think is quite likely.
Amother possible drive might be a small electric generator powering a moving coil meter for the display but I think this is less likely.
Ced.

R1100RT 1996.
Sent by Boson Quantum Transmission from the Starship 'Galahad'.
http://www.researchgate.net/publication ... ing_bosons" onclick="window.open(this.href);return false; - It works!


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