XJ750-J Speedometer reads slow.

Hi all, I finally got this thing out on the road today for the first time as I haven't had it long and noticed I was going faster than the speedometer was reading. I stopped and turned a gps speedometer on my phone and up to 20 mph they read almost identical, but from there on my phone starts leaving the speedometer behind. I get to 60 mph on the gps and the speedometer is showing between 40 to 45 mph. Any thoughts? I tried to find a thread on this but could not. Thank You.

 

Could be that the instrument has too much damping for the needle, causing too much friction and a false reading. Or the magnet has lost some of its power. At least it unlikely that the fault is outside the speedometer - unless the needle is bouncing significantly. It is not unusual for old instruments to show various faults as they age, either from knocks or other factors, including wear and contamination.

The way a mechanical speedo works is quite simple https://www.howacarworks.com/accessories/how-a-speedo-works

 

Thanks faffi. That's interesting information. I had a bad feeling that I was going to have to replace it. I have to find one now. There are some on ebay and none of the sellers will test anything before they sell it.

 

There are good odds that any OEM speedo you find will also need to be calibrated.
Cheap solution (my preference) use a lable maker to re-lable the speedometer based on your GPS.


Not at all cheap solution is to send it out for recalibration.
It's somewhat possible to do at home, but it's tricky and my results have been less than satasfatory.

Middle-cost solution is to fit an afermarket speedometer and either ditch or modify the OEM gauge cluster.

 

Thanks k-moe. My speedometer doesn't get above 45 no matter how fast I'm going. The renumbering would be great if it weren't for that. I'll probably manufacture my own phone mount to use the gps. If it ruins the phone then so be it, it's insured. Thanks for the help.

 

That's interesting - I would suggest a bit of debris (rust) has got wedged in the movement. Not sure if this is rebuildable?

 

There’s a thread/howto regarding cleaning the speedo mechanism. I did it last winter. Let me find it…..

 

Here is the thread I was referring to: https://www.xjbikes.com/forums/threads/speedo-and-tach-dissection-with-pics.15303/#post-126943

I used it as a reference when I did my speedo last winter. I did a complete overhaul of the system. Clean and lube gear unit, new "clutch" pieces, new cable, cleaned the mechanism as described in the post. Works like new now.

If you have a calibration issue, then all the cleaning in the world is not going to help, I suppose.

 

I just tried cleaning it and it did not make any difference. I did find a used one in nice shape but it hasn't arrived yet. If that doesn't work it's time for a phone mount to use the gps.