[Rick Parker]

PMILLER176:

Aside from the tachometer having an internal dampening mechanism to keep the needle steady there are many others things that could cause what you are experiencing. The tach is controlled by the primary side of the ignition, that is the triggered 12 volt source. Check all item that affect the primary side. For instance, ignition switch, grounds, including the one to the MSD box (if used), wires and connections to the coil, the field coil inside the distributor is a prime suspec as it warms up from engine heat or current flow if it has a frayed winding or some rust, the resistance can change and cause this. Lastly an internal part of the Tach could be failing. Stewart Warner in particular were known for this malfunction. Good luck