I do not know this type of ignition but why insist on the ignition ? Does
it have a belt driven fan ? You can read that using an opto. You can put a
Hall sensor on one of the coils, you can use a proximity sensor on the
clutch/starter gear disc etc. Once it is clear that the ignition is not
tinker-friendly, why try harder to interfere with it ? Other
electromechanical pickoffs for tachometers are vibration of the engine
block and camshaft(s) and sound of same. A microphone in the intake
manifold will also work. It requires some filtering but the suction is
unmistakably the highest signal generated (several tens of millivolts
using a normal dynamic microphone capsule). This is the only way with
small diesel engines that need to be modified.

Peter

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