Tach from spark wire...

[Bohdan L Bodnar]

How about wrapping several turns of small wire around the ignition coil,
running this into a pulse amplifier (e.g., an N-channel JFET with drain tied
to Vdd with a high value resistor), run the output into a divide-by-two
(you'll get two pulses per cycle on these lawnmower-type engines), run the
output into a power amplifier, run this into your tach?  If you're going to
"custom-design" a tach, you can get rid of the flip-flop divider and go
directly into the tach.  The usual analog tach is nothing more than a
monostable multivibrator driving driving a crude integrator whose output is
read by a voltmeter (i.e., an ammeter in series with a resistor).

Bohdan Bodnar