Speedo cable reading 20kms slow

[zelto]

speedo is reading 20kms slower than actual speed, after pulling apart , inspecting (looks fine) lubricating and reassembling, it is still reading slow, been this way for 5,000kms.No worries replacing it, but as a matter of interest, what may be causing this roswell incident? Any wild and wooly theories welcomed!! cheers

[imdying]

Is it driven from the front wheel, or does the cable go down to the motor/gearbox on that model?

If the front wheel, then you'll probably need someone like Parrot & Son to fix it. If the gearbox, have you changed the sprockets/gearing at all?

[zelto]

do you have any idea what Parrot & Son would actually do, ie what is causing it.If there was a wrong part installed by previous owner, seems weird that it fits.Guess i will admit defeat and pay the proffessionals, it goes against my religion though, i want to fix my own bike.cheers for the help imdying

[Jantar]

Is the error 20 km/h right through the range? As in does it read 0 until you are actually doing 20 then start to climb?

Or is it 20 slow at a particular speed, but with a linear error?

If the first symptom, then I would suspect that the needle has been off at some stage, and hasn't been replaced at the correct angle on the pin. If the second symptom, then there is possibly an incorrect drive in the front hub, or the speedo internals are damaged.

[T.W.R]

When you say it's been like this for 5000km it starts to narrow things down a bit :yes:
By chance this speedo error didn't start after fitting a new front tyre? A larger aspect ratio will increase a speedo's normal calibration error and with a 21yr old bike the error margin will have grown anyhow so any more variation will increase the margin. The error margin is a percentile figure of actual speed compared to indicated speed
BTW standard tyre is a 120/80 x 16

Silly question time though; the standard front wheel hasn't been replaced with a 17in wheel at all? Only because of the size of the error reading and from what you tell as a consistent figure.

[imdying]

The guys are right, you want to figure out what the rate of error is. Does the needle sit at 20km/hr when the bike is at rest, or does it get increasingly out? You need to find a gps unit or a decent car to map it against. That'll help them figure out what needs fixing, saving $$$. They're pretty good though, they rebuild and restore automotive instruments as their speciality. Of course if it's out at a constant ratio, there's something to be said for getting one of those inline cable speedo correctors (like a speedo healer for old bikes). No idea what they're worth or where to get them (but I'd start at Parrots)... but on an 86 GSX750 I'd just live with it until the bike dies myself :blip:

[zelto]

some good ideas here guys,thanks.To be a little more specific, i have had the bike for 3,000km -so it was doing it when i brought it-the previous owner said it was doing it for a ''while'' so its difficult to know the exact point it started, allthough i am going to talk to the previous previous owner.As for constant or gradual, i will go for a ride tommorow and get definitive on that, i cant recall riding at 15kmh with the speedo reading zero though, but that would point to needle/stem and make it easier for me to fix.I am pretty sure it is gradual/linear/percentile, but will test to confirm.The bike has stock standard everything incl 16'' front wheel.

I mean its not a big deal, but i have gone through the bike and got it right up to scratch, and this is the last of 20 jobs i have done on the bike, so its that buzz you know , old bike all sorted, done all myself running sweet, and satisfaction of fixing the finnackey easy to ignore little jobs, like i just remodelled the rubber receiving washer that a peg from the sidecover goes into, because it was loose and not snug.As for ''... but on an 86 GSX750 I'd just live with it until the bike dies myself'' i missed a breath at this, lol, dont these bikes last forever no more!The bike has 45,000 on her and buckets of compression, i reckon it will get to 100ks before recon, thats another 10 years for me, weekend/summer riding.
If test indicates problem is gearing, i will pull it to bits again and have another look , then ask the pros what they think, if its not obviously broken/bent or out of align, then its bigger than me.Will let you know, cheers again for the input.

[Pancakes]

You need to get a longer cable, Is it just the front wheel or both going 20k's faster?