Is the rev counter really that fussy? I've only had one early 1850 and that was my first Dolomite in the late 80s, it stayed firmly on points! But presumably this car DID run with the EI feed scotched to the tacho output (A horrible bodge but still....) and the OP didn't mention the tach not working.
Yes. The RVI types do sensor the current feed pulses to the coil, turned on and off by the points. Connecting another current user on coil positive, weakens the pulses as there's a constant user connected also making the tach not working. When you have an RVI tach/ignition system nothing extra should be connected to coil positive as it's not just an +15.
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