XJ700N Tach Doesn't Return to Zero

I recently purchased a 1985 XJ700N Maxim that needed restoration. After a fair amount of work, I got the engine running. Initially, the tach worked fine. Now, after some non-engine work on the bike, the tach still responds to the throttle ok but never returns to zero with or without power on the bike (see photo). I took the tach apart to make sure everything works ok--everything looks fine and the needle swings easily, it just always returns to a value between 2000-4000 rpm with no power applied to the tach. Applying power doesn't return the needle to zero. Any idea what's going on here?

 

And when the engine is running it still stay 2000 and above, even at idle? Makes one wonder if the tact is correct at higher rpm's. On my 85 XJ700N, at 3500 rpm the speed is about 46 mph. You can use that to verify the correctness of the tach.

 

Thanks for the response, tabaka. Yes, when the engine is running, the idle speed is about 1000 rpm above the tach indication when the bike is not running. IOW, the idle speed on the tach indicates 3000-5000 rpm. I can tell just by how the bike runs that the true engine rotation speed is about 1000 rpm, not the indicated 3000-5000 rpm. So, something is wrong with the tach. That said, the tach was fine when I started the project, but now it's off by 2000-4000 rpm. I don't know if the tach just went bad, or if somehow bad/corroded connections have created a problem with the tach (magnetized or demagnetized the tach to cause an error in the indication??). If the later, I'm concerned I'll just damage another tach unless I fix the root problem first.

 

I have never looked at mine, but some tachs have adjustment screws to fine tune them. I'm wondering if the needle has just slipped on the shaft. I've had that happen on clocks when the shaft to hand (clock) isn't very tight. You might want to check that. Have you driven the bike? If so, see if you are doing about 45 mph at 5500 rpm. It should be 3500 rpm. If you are getting 5500 rpm I would strongly suspect a loose needle.

 

No adjustments on my tach, and the needle appears to be very secure on the shaft. The bike is not jet rideable, but when it is I expect to see the tach around 6000 rpm at 45 mph.

 

May be time to start looking for a new tach. Strange.