Bill;

One common cause of generous (early 1800) tach readings, is a poor connection of slider of its calibration potentioneter. (Carefully) drill access hole, mark slider position angle, apply a drop of Deoxid D5, rotate slider back and forth a bunch of times to clean and spread the D5, apply another drop, return slider to original angle (recalibrate if you need to have it perfectly accurate, but do you really get that close to read-line?), reinstall and behold the fruits of your accomplishment and workmanship!

I haven't yet tried to get anti-corrosive paste into this pot, but would expect this to work even better, as it gives a gas-tight-joint on the slider connection whereas the Deoxid will eventually need to be reapplied.

Alternative: Open tach completely, measure pot, slider position with Ohm-meter, and replace that pot with fixed resistors which will NEVER AGAIN get intermittent (opening up the tach requires beating up the case pretty good though)...maybe not such a good idea.

See also: Gas-Tight-Joint tech article and Service Notes section for more details, link below.

Cheers