The tachometer doesn't directly count engine revolutions...it counts pulses from the alternator. So my first thought is that you have an electrical problem, possibly a bad connection, either at the alternator, or even in the engine instruments with a wire connection or a bad ground.

First step, I think, would be to do a physical inspection of all wiring connections, especially looking for a loose screw or a bad crimped terminal.

FWIW;
Mine has done this whenever I either: leave the dock with a full charge or
after an extended time motoring, (when the batteries are approaching full charge). I have a Balmar alternator and their "smart" regulator.

It drove me nuts enough to install a switch to kill the tach when the "bounce" happens. I do NOT kill the alternator itself.

Jeff,
By your hull number the first thing I would suspect is the molded eight-pin connectors in the wiring harness between the engine and the engine panel. They are notorious for creating phantom electrical problems. Have you done the wiring harness upgrade to the euro-style connectors?

Wally, when the batteries are full the smart regs go into float and they usually turn the power right off or off and on for some period of time while dropping the volts and amps down to the float settings. This results in the tach either going to zero or bouncing around a bit before it settles down again. Both makes of smart regs I've used do this briefly for maybe up to 2 minutes, but then it settles down.

My recent eractic tach problems were caused by a slipping belt. Check for black dust on your altenator, cracking or glazing on the belt itself.

[QUOTE=bstreet;9179]Wally, when the batteries are full the smart regs go into float and they usually turn the power right off or off and on for some period of time while dropping the volts and amps down to the float settings. This results in the tach either going to zero or bouncing around a bit before it settles down again. Both makes of smart regs I've used do this briefly for maybe up to 2 minutes, but then it settles down.[/QUOTE]

You got me thinking, so I called Balmar and they assured me that the bouncing needle is a normal condition. The fellow said it's voltage driven and a full charge will cause this condition. He went on to say that the newer regulators, (mine is an ARS-3), have an adjustment for the float voltage and you can boost it by a tenth and that should take care of the problem.

BR;
Wally