Tacho goes a bit nuts at ~80mph...

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Dont know where to put this so its in here.

I go up north about once a month to see my wife and the bike runs fine, but i have noticed a "quirk". When i do 70mph (thats speedometer reading, its actually about 66mph), everything fine.
When i do 75mph (a true 70mph) everything is fine.
When i hit an indicated 80mph on the speedo the tacho swings wildly from 6000 to 8000rpm, its usually doing ~7000rpm IIRC, is this something i should be woried about? Or is it most likely a cable coming loose, or a sensor thats about to go FUBAR?

It doesnt do it in any other gear at 7000rpm as far as i can tell, although i will test this tomorrow on my way to work (traffic permitting)...im just interested to know peoples opinions on what could be going wrong.

Cheers

[RiceBurner]

Is it a UK bike, or a Japanese import that's been derestricted?

I'm guessing here - but if it's a Japanese import, the de-restriction may have something to do with it.

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UK spec im afraid, traffic was to busy today to test it at 7000rpm in different gears, will try tomorrow...im wondering if it just needs a cable tightening?

[Tirpitz]

You haven't told us what gear you're in. If you're in top each 1000rpm equates to about 10mph, so at an idicated 80mph you should have 8k on the tacho, not 7k. Changing gears won't make any difference to a fault with the tacho, there is no link between the gearbox and any electrics. There may be more vibration in a particular gear which could cause the actual problem to reveal itself.

If a cable is coming loose then most likely you will see the needle swinging about all over and not just at one speed. however, check the wire run back from the tacho all the way and check carefully for chafed insulation or trapped wires.

So what happens above the problem area? Once it starts swinging about have you tried increasing sped to see if it continues at higher revs.

It's most likely loose / chafed wires or a tacho which is on its way out. You'll have to eliminate the first before doing anything else.

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sorry, top gear, tested it the other day there is no swinging of the needle at ~7k rpm in either 1st or second, dont know about the others.

its in a really narrow mhp range, at 78mph it starts and stops by 82mph...which makes it all the more strange

its more of an annoyance that anything else, but ill check the cable-routings tomorrow.