I have a similar kind of problem - built the circuit and it works, but I'm not 100% sure why
I've got an old suby with (I think) an electronic distributor.
I built the circuit as for a "points" style ignition, because I don't really understand what the difference between that, and the one I have is - shouldn't they all just swing the coil supply ?
As described in the 2917 datasheet, there's a diode on the negative comparator input (pin 12), which holds it at 0.6V.
The input then gets 1/3 knocked off by a voltage divider, and is fed into pin 1 with a 0.02uF cap to ground.
I measured the input at pin 1 about 8V DC, and 3 V AC on my DMM (not sure if the AC is RMS or not). This was not triggering the tach.
I added a diode forward biased towards pin 1 (ie. current can flow from coil to pin 1) and bingo, it worked. I'm kind of curious why ...
I'm just not sure how to make sense of the readings I was seeing, and why the diode fixed it.