One of my tacs decided to get stupid today. It is fine up to 3200 RPM, but after that it bounces about 300-500 RPM after 3200.
What would cause that? No changes other than new plugs & wires yesterday on both motors.

just a shot here, could you be getting interference from a plug wire to close the tach line at higher rpm's ?

Mine did that when I had a loose engine ground wire.

Look for a loose ground. I had two previous boats that did it all the time. I would tap on the glass of the tach and they would settle back to normal.

x2 Bad ground.

Chris

I had the same problem and my purple wire (tach signal wire) came loose from the coil. It was on there, but it wasn't really tight

I'm guessing that is the deal or interference from a plug wire. After the temperatures get under 100* today I might go out & take a peak.

Thats how I understood it.... I swapped it out to a tach that had a filter and a steel case and it got much better, but it was definitley a "radio frequency" type interference.... eventually found the signal wire loose... and it wasn't obvious.... cleaned it up and tightenend it down real good and that was it for me, problem solved.... I posted a tyhread a while back on my issues, I got a few pages of GREAT suggestions... look it up!