I've had some luck reducing ignition noise on 2M in my MGB by putting a few
'clip-on' ferrites on the low tension side of the ignition circuit. (The
tachometer uses a current sense rather than a voltage sense, so the raw coil
current runs into the cockpit.)

Would ferrite also help on the spark plug wires? BTW, this is an old car
with points.

--
Bob D. ND9B