I think a bouncing tach is caused by friction in the cable or tach head. You can loosen the cable at one end (I forget which end), take out the core, inspect it, and grease it before re-assembly.
I went one step farther on the prior YM186D. It's tach was screechy when it hadn't run for a few days. I took out the tach, turned it face down, and poured in a few drops of 'lock fluid' (graphite in solvent) around the edge of the socket that the cable turns. Then let it sit overnight for the solvent to dry. That solved the problem, the tach was then steady and silent like new.