I have 1990 454's with thunderbolt ignition. I put new v.d.o gauges in and one jump's around anything under 2000 r.p.m above it is fine. I switched tach's from port to starboard and get same results with both tachs. does the tach pickup from the coil or the ignition module? Thank's for any imput.

From the coil ( -) Gray wire.

I had the same problem with a pair of 525Sc's , I found the rotors were loose on the dist shaft. Merc says to locktite the rotors on. let set for 24 hrs. this fixed mine. good luck.

I have a single SBC in a Volvo Penta boat and had tach jumpping all around and traced it back to a poor connection. I dunno if it was at the tach, the engine connection or the coil. I cleaned all of them and it fixed it.

Ditto to what I had.