[PANAVIA]

Don , I am assuming this is the Smiths Repro tach.

Question ; Does the tachometer track at other RPMs >? (idle -3000 )

I am wondering if you are having AC voltage feedback at high revs with the Alternator.

my other idea would be the tach feeding off of the same positive circuit as the MSD.

Do you have a another tach that you can test with >?