Len,
A fellow club member has the same problem (Unique + SW). My older Unique with a 20 year old tach 'was' working fine, but now starts reading high at times..... and rapping the glass with my knuckle seems to restore normal ops.... suspect something loose inside, but that's another problem.
Getting back to my friend, I put his tach on the bench and tested it with test equipment and the thing was accurate to within a few percent all to way up the scale. Went over all the wiring, paying attention to grounds and ground loops..... everything looks good. I suspect that there is something about the "signal" from the ignition system that the tach gets confussed over, hence the "tach adaptor" that's been talked about. It must condition the signal so that the tach sees a "cleaner" signal.
My tests have shown that trying an old fashioned points distributor makes the problem go away.... something to be said about compatablity problems between old and new stuff
Like you, I'll be looking back to this thread for solutions.
- Jim -