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.....

[4129R]

This TCS was running perfectly last summer, but just would not start this spring.

I tried all the usual things, then replaced the coil, then replaced the distributor, and still FA, it just refused to start, with no signs of life.

Then I thoroughly cleaned all 4 spark plugs with emery paper between the gap where the spark is generated, and the engine sprung into life second time, with just 3 throttle pumps on the 40DCOEs.

Strange how something so simple can be so hard to trace. All 4 plugs were sparking properly when placed on the rocker cover, but not apparently when inside the cylinder head.

Now at least I have the engine running to get all the fine adjustments done before the car goes to an ex Classic Lotus paint sprayer to get the paintwork sorted. Lots of 3mm bubbles on the front wings, and the whole car needs to be polished to make the paint shine.

[jbcollier]

Oxidization of point surfaces is pretty common, especially on points that have been in service for a while. The special coatings “wear” away and points become more susceptible. Using abrasives to “clean” the points will accelerate their ultimate demise.  Use clean, white, paper to clean control points.  Paper is abrasive enough and will not cause further damage.  Being white, it allows you to see how much of an issue there was, and, when it pulls through clean, when you’re done.

[4129R]

The new distributor had new points, condenser and rotor arm fitted, with the correct gap on the open points. All I had to do was fit it with the timing correct.

I find removing the carbs makes working on the dizzy much easier, especially access to the 7/16 bolt tightening the fixing collar, and the 7/16 UNC bolt holding the collar to the block.