What it does – 

1. Signal from the points goes into the 1k resistor at the left.
2. 1 uf capacitor and 12v zener clean out the spikes from the ignition to a rather clean 12v pulse.
3. CD4011 further cleans the pulse to a good looking squared corner pulse.
4. CD4013 is a flip flop – it takes the incoming pulses and puts out 1 pulse for each 2 pulses that come into it. First it flips – then it flops. The first pulse in makes it go high, then the next pulse makes it go low – essentially dividing the incoming signal by 2. That is what makes the tach think the engine is a 4 cylinder.
5. The reset of the circuit is just clean up for the tach input. You could leave off the LED and 2.2K resistor – they’re just for show. The three 2.2k resistors in parallel will handle 3 times the current of 1 – I only had 1/4 watt resistors available and thought the tach may need it – but now I’m thinking they were not needed.
6. The 2.2k from the 12v batter feed along with the 12v zener and 10uf cap just provide a cleaner 12v source for the 2 chips.

I rad this circuit for a few years with no problems at all but then ->

This is the first tach circuit I put together. It basicall takes a pulse from the distributor and outputs two for each one in. It worked fine but then with the help of the net came upon a better one with almost no parts (one pot or volume control) .