XJ900 Seca erratic tach

Still sorting through the issues with the "new to me" 83 XJ900 Seca. Currently working on the tach which bounces around and swings wildly. It also tends to increase reading with time. Last ride it eventually got to a reading of 9500 RPM at a constant 70 MPH. Not sure if this is an electrical issue or a tach mechanical issue. Any thoughts?

 

Probably an electrical issue since it is reading higher than it should - do you have another tach that you can try?

Is the needle steady at zero with the key on and bike not running?

 

The needle reads zero with the ignition on but the engine not running. It is actually pretty stable with the engine idling but starts swinging when the road speed picks up. It becomes somewhat stable again when you hit about 50 MPH in top gear but this is when it starts creeping up (I saw 9000 RPM and it was still climbing but I had reached my destination. I don't know how high it might have gone.) I have looked at the wiring but the wiring on my bike doesn't match the wiring on the diagram. I have the two wires running to the " reed switch" and to the tach but do not have the Green and yellow wiring that should be coming from the "ignitor". There does not seen anyplace for this wire to land so I doubt it was ever there. No Green and yellow wire at the ignitor end either.

 

As in Gy? That would be the grey wire, which is a spliced ignition wire to provide an input to the tach. The reed switch is part of the speedometer and is used by the auto cancel unit to automatically turn off the flashers - it's confusing on the diagram (if we have the same one) as it looks to be part of the tach -item 43. Item 42, the rev limiter per Chacal some 900's are equipped with, but I have no clue which one's.

Never a bad idea to check the connections, in this case the 9 pin connector inside the headlight bucket that connects to the tachometer.

A light whack on the side of the meter might be beneficial - if it acts up then suspect a poor connection inside.