RPM tach signal fluctuating. Yamaha 40 BR

Hi guys, i have a following problem on my 1978 Yamaha 40 BR

From the stator there are 2 cables. 1st cable has 3 wires(orange/gray for ignition coils, black for ground), 2nd cable has 2 green wires(for charging circuit that goes to voltage regulator).

I've connected the tachometer to 1 of the green wires to measure the HZ from that contact breaker platinum ignition system and issue is that it doesn't hold steady frequency. On steady throttle lets where it should stay on lets say 100hz it reads +-30hz and just oscilates up and down which throws off my tachometer readings.

My tach is digital waits 3 seconds and then outputs value and it jumps up and down 500-600rpm.

Is there something i can do to make it work? I know analoge guage might work better but signal still oscilates way to much to filter it correctly.

Any ideas are welcome