1986 B-28 - 260hp Mercruisers Starboard Engine Tach issues

[JeremyD]

Sometimes when I am starting the starboard engine the tach will register 3500 rpm's - on July 4th It settled at 1000 rpms' idle - then when I powered up - tach swung to 5000 rpm's (obviously not there based on speed/throttle placement)

After I stopped the engines to watch fireworks - it worked normally - smooth idle - accurate tach.

No loose connections on the back of the tach. Coil recently replaced (had a spare)

Does the tach get it's signal from the alternator? That shows 14 volts pretty consistently.

Thunderbolt ignition amplifier going bad? (every once in awhile the idle will surge up and down)

I know my way around a flat 6 - SBC not so much.

Thanks in advance.

[MarkD]

Jeremy:

Mine do the same thing. Often times if I give the tach a few taps, they settle down. I'll be interested to hear what the others say ... I wonder if it is related to my lack of spark on the starboard engine.

[Leigh]

I have a set of Faria tachs on the upper station and the port side jumps around at low RPM's. Settles down once you push the throttles up. Has done it since I bought the boat in 2005. I don't think it's connected to any ignition issues, as every component of the ignition system has been replaced at least once since I've owned the boat. Leigh

[Craig Mac]

Just went through this on my new VDO guages----starboard tach was all over the place---eventually replaced ignition module and it settled down but still bouncing a little---VDO has tecnical bulletin on erratic operation-suggests to put a diode in line---just did that and bouncing stopped---did not sea trial.

[JeremyD]

Thanks - maybe I won't worry about it as much - appreciate the feedback though.