I run a 4AGE silvertop and have "strange" behaviour on my tacho. I am not using the aux wire for the tacho as that is being used for other purposes and have the tacho wired to the coilpack. The results I get on the tacho are when running idle show "dancing" between 800 and 2000 revs.

Can anyone help me with this?

Thanks...

extra info, i am using an OMEX 500 ECU...

That's probably interference from the energized coils into the wire that feeds the tacho it's signal.

You can try to put a coaxial cable between the tacho and the coilpack.

you could take a good/ish type of of microphone wire or even headphones wire for that. connect the shield to the tacho's earth (only connect 1 side of the shield NEVER both because of earth loop)
and the inside wire to the signal ends of the coilpack and the tacho itself.

if that doesn't do the trick your tacho might be broken, or it's signal wire might need capping of with a few picofarad to balance the low rpm signal (like in old car radio's)

grtz Thomas