Bouncing tach at idle
 

We've all heard the "bouncing tach" as an indication of a failing voltage regulator. Let me throw in my 2 cents.

Recently, my 82SC had developed the "bouncing tach" at idle speed. 900-1100 rpm and no noticeable change in engine rpm. Not long later, I began to notice a "crunchy" sound from the engine. Narrowed it down to the ignition distributor. Put my hand on the fan housing while the engine was running and smooth as silk. Grabbed the distributor and noticed a lot of vibration.

Removed the distributor and noticed the up and down movement in the shaft was in the order of 2.1 mm and the "spider" is starting to wear down into the plastic of the coil. Obviously the fiber washer under the rotating plate in the distributor is toast. The vacuum advance held a good vacuum but not rotating the pick up inside the distributor. Swapped it out with a "loaner" known good distributor for the time being and all is well again. Engine runs much better and the tach at idle is steady as a rock.

So before you go down the voltage regulator replacement path, check your distributor!