I have a vintage Sun tach from the 1960's, just won't read at high rpm's, it actually the needle goes the oppisite way the higher you rev it, works well at low rpms. Mike has the same tach and MSD, his does the same thing too. I was told MSD may have a filter that will allow the tach to work, otherwise I need to have the tach or circits rebuilt or updated and it will work.