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Tach Needle spins in circles

N

Nathan Stanley

Just got it firing!
Joined
May 10, 2020
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Location
Las Vegas
I’m reviving a 1975 850T after a short 8 year storage period.
I’m troubleshooting several issues, but my skills/knowledge only take me so far (Haynes manual just arrived). Without throwing money and a bucket of new parts at it, I can’t figure out the tachometer. The needle rotates clockwise 360 degrees about 1 full rev per second at idle and speeds up with engine RPMs.
I took the cable off and everything looks normal at both ends. The PO slopped a bunch of heavy grease in the instrument end of the cable which, I assume, is inside the tach instrument housing now.
Any ideas for what might be causing the needle to rotate all the way around?
 
If memory serve me correctly, the needle is magnet driven against spring tension. If enough grease got up there instead of the magnet driving the needle, the grease is effectively making the system a "direct drive" If you can carefully disassemble the unit and clean the grease out you may be OK. At this point the gauge is useless so nothing to lose. Harper's shows a new unit for $192.01 if available. harpermoto.com/tachometer-14767201.html
 
After “carefully” removing the top bezel and glass (now cracked... oops) I was able to inspect the guts. The copper return/resistance spring for the needle was broken and bird-nested around the needle shaft.
 
Nathan Stanley said:
After “carefully” removing the top bezel and glass (now cracked... oops) I was able to inspect the guts. The copper return/resistance spring for the needle was broken and bird-nested around the needle shaft.
Click to expand...

That explains it. Haven't hear of that spring breaking before but anything can go wrong.
 
Nathan Stanley said:
After “carefully” removing the top bezel and glass (now cracked... oops) I was able to inspect the guts. The copper return/resistance spring for the needle was broken and bird-nested around the needle shaft.
Click to expand...
That spring is called the hairspring. The drag generated by the spinning magnet inside the aluminium drag cup is calibrated against that hairspring tension to give you a reading. Even if your pointer did only 1 full revolution that spring has completely had it. It can be replaced and the tacho recalibrated, but that would be completely cost prohibitive imo. There are probably more things wrong in there that allowed this to happen in the 1st place.

Things can wear in there and eventually allow the the spining magnet to physically contact the drag cup. 1 touch and it's all over for your hairspring.

It's entirely UNlikely that enough (or even any) grease made its way into your tacho. If it did, there's another problem with your instrument. In fact, all you blokes with cable driven instruments, a light smear of grease the full length of the inner cable plus a little more grease at each end before reassembly is a very good thing to do.

I'm a long retired old school instrument fitter (Scientific Instrument Maker and Repairer) and the higher quality versions of these sorts of things are some of what I used to work on. Ex-RAAF aircraft instrument fitter.
 
john zibell said:
Yes he is an old codger, but I don't think he fiddles with gauges.:giggle:
Click to expand...

Nope. But he’ll tear a bike apart, throwing stuff here and there and he always manages to have extra nuts and bolts when he’s done! It’s amazing.
 
I know plenty of ol' codgers - heck, I are one! But I don't know any doing this king of stuff. Not on this side of the country anyway.
 
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