Faulty Tacho - Misfire ? Hard to Start ?

[4129R]

I have a faulty tacho, which does not read high RPM.

The car is hard to start and does not want to fire cleanly above around 3000 rpm

As the white wire from the ignition switch goes to the tacho, then on to the HT coil, would a faulty tacho give intermittent power from the tacho down the white wire to the + side of the coil and make it hard to start and misfire at high RPM ?

On reflection, I think I will short out the two white wires going into the tacho and see if that makes any difference to starting the engine and the misfire.

[BDA]

As the white wire from the ignition switch goes to the tacho, then on to the HT coil, would a faulty tacho give intermittent power from the tacho down the white wire to the + side of the coil and make it hard to start and misfire at high RPM ?

I doubt it since I think the tach is just counting the voltage drops or spikes of the coil but you could test it by pulling the white wire that goes to the tach off the coil and seeing if that helps.

[EuropaTC]

As BDA says, the white wire is just a loop to measure pulses. A poor connection might cause an issue but if there are problems above 3k then assuming it's ignition I'd look at coils and if you're still on those quaint things called contact breaker points, them & the condenser.

Brian

[4129R]

As BDA says, the white wire is just a loop to measure pulses.
Brian

No, the white wire provides 12+v from the ignition switch via the tacho to the + side of the HT coil, so the tacho counts the number of times the points open and close generating the HT spark from the coil.

[BDA]

looking at the wiring diagram, I see you're right that the ignition wire goes through the tach to the coil. I guess I was confused because when I was trying to convert my tach for a Pertronix ignition, an extra wire was added which I could use to  disconnect the tach from the coil as I suggested.

I think I remember now that inside the tach, there is a coil of wire that surrounds that what white wire going to the coil and I believe that coil of wire is what actually provides the signal for the counting of the pulses of the coil. In any case, you should be able to bypass the tach and give current to the coil directly. It may require cutting a wire and adding some spade connectors to make it a connection you can disconnect.

That is admittedly from the little memory I have left but I think it should work for your test of a tachless car.

[EuropaTC]

No, the white wire provides 12+v from the ignition switch via the tacho to the + side of the HT coil, so the tacho counts the number of times the points open and close generating the HT spark from the coil.

Hmm, memory going again then. I don't know if it will help any but here's a photo of my old one when I changed over to modern internals. I thought it was a continuous wire loop but this was taken a long while ago.....