I removed the speedo/tach, and bench tested the tach needle using a hand drill going in reverse. I drill could only get it up to 3000 rpm's, but it was jumping around around. The tach is a very simple worm gear mechanism. Two white plastic worm gears drive the main needle. I sprayed some silicone around both gears, then applied silicone grease to one of the white gears. I but it back together, and then retested on the bench, it did operatre much better.
One thing to note, I was bench testing with a spare cable. The tach cable on the bike is of unknown age and might have something to do with the issue. When first bought the bike, the lower end of the tach cable where it plugs into the bike near the cam, was dirty with oil. I did clean up that area, but never removed the cable. If this still has problems after road test I will replace with my spare cable and give it go.
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