Need help fixing large speedometer error on 1984 RZ350

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garcenw
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Joined: Tue Nov 17, 2015 12:44 am

Need help fixing large speedometer error on 1984 RZ350

#1 Post by garcenw » Wed Jun 21, 2017 11:45 pm

I finally got my 1984 RZ350 restoration project road worthy. I've noticed that the speedo reads fast. I used GPS to calculate the speedo error and it's response slope is ~16% too steep across the board on top of an offset of ~3.3 mph. I also checked the odometer and its error is only off by ~2%. The link below is a graph with a linear regression fit of the data points.

https://www.dropbox.com/s/yat4kp2spsjhz ... r.jpg?dl=0

I suspect folks will ask, so here is some additional info:

1) Tire sizes are correct (which supports the odometer being close to correct).
2) There is no issue with wrong gearing on the speedo drive at the front wheel (again, the odometer being close to spot on supports this).
3) The speedo cable turns freely and is well lubed.

I can easily move the gauge needle on its axle to change the offset, but that just moves the error around as it only changes the preload on the gauge needle’s return spring. It would have the effect of simply moving the line in the graph above up or down in the y-axis. For instance, I could choose to make the speedo read correctly at 60 mph, but the error would grow as I move faster or slower than 60 (under-predicting speed for speed lower than 60, over-predicting for speeds greater than 60). I’m looking for other options. Anyone out there have any experience messing with the inner workings of the gauge to get it to operate with less error? I don’t really want to get another gauge on EBay as I don’t want an odometer discrepancy.

Thanks.

- Walt

garcenw
Posts: 80
Joined: Tue Nov 17, 2015 12:44 am

Re: Need help fixing large speedometer error on 1984 RZ350

#2 Post by garcenw » Mon Jun 26, 2017 9:13 pm

I decided on the mechanical fix alluded to above. I rotated the needle on its axle. I choose to make 40 mph on the gauge to match 40 mph on the GPS, going with the assumption that my phone's GPS is reasonably correct. I tabulated the various expected outcomes for the various offsets from baseline and those are in the table linked below:

https://www.dropbox.com/s/wziov9wd404q1 ... t.jpg?dl=0

I chose a -10 mph offset. After putting a -10 mph offset on the gauge, I checked the speedo again with the GPS and was within 1 mph of prediction in the speed range I checked (indicated 20 to 70 mph).

silverstrom
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Re: Need help fixing large speedometer error on 1984 RZ350

#3 Post by silverstrom » Sun Jul 02, 2017 5:32 pm

Most Japanese analogue speedometers read incorrectly. I have a 2007 Suzuki and it reads 8% high and it is a well known issue. The farther back in time you go, the more the Japanese high reading error is an issue. I contemplated using my GPS to check all of my bikes, but decided it wasn't worth the grief it would uncover. Ignorance is bliss, and all that. I'm quite happy not knowing how inaccurate my smokers speedos are. The speed seldom matters.

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