Good enough info in that PDF it seems like it should be on site somewhere so here it is:
Electronic Tachometer Adapter
As most of us figure out at some point the factory tachometer on most older bikes isn't very accurate at low speeds and tune up time becomes an impossible task trying to adjust the carburetors or anything else that requires reasonable accuracy. On my CB750 the idle speed is specified to be ±100rpm, choke speed ±200 rpm and the pilot adjustment you have to look for a change of 50 rpm or less! Needless to say the factory tachometer just doesn't cut it and installing an electronic tachometer will only read the pulses from 1 coil so the reading is cut in half. Having a tach-dwell meter left over from prehistoric days I decided to find a way to utilize it on the dual coil system. Below is the circuit I came up with to adapt the dual coil circuit to use an electronic tachometer.
R1 can be any ¼ watt or larger 10k resistor. D1 and D2 can be any 4000 series diode. The two diodes form a simple and gate which will allow each coil to trigger the tachometer without interfering with the other coils operation. Although I marked the leads to the coils for coil 1,4 and coil 2,3 it really doesn't matter which wire goes to which coil as long as they go to the – side of the coil. I used an alligator clip for the battery connection and soldered together a male and female butt connector to make the coil connections.
© Jerry Rotert 5-2009
Update: I received a few questions about the hookup do here are a few pix. Hopefully this will clarify some things.
For the splice I started with a male and a female butt connector and spliced them as shown. The wire in the right picture goes one of the diodes. You will need two or three of these splices depending on your application. Note: I actually removed the blue insulators and soldered the connections for durability but they can also be crimped if you wish.
Here is the connector on the left coil. You will need another on the right side. The pix on right shows the complete cable that I made. The 2 diodes and the resistor are inside the shrink tube. The 2 yellow wires marked coil go to the diodes in the schematic. The purple wire goes to the tachometer and the red wire goes to the battery. Someone ask about making it permanent for a regular tachometer. In that case you would of coarse want to make the wires as short as possible and connect the battery wire to a switched connection. This could be done by making a third “tap” and connecting to the + side of one of the coils. Since I just made it as a tune up aid I used an alligator clip.
Note: you can make the wires any color you want. I just had these colors handy. The 2 yellow wires go the the – side of the coils. The red wire goes to the battery or optionally to the + side of one coil. The purple wire goes to the tachometer. The ground wire from the tachometer connects to any good ground.
