My Vincati has a electronic Smiths tachometer fitted and a B-TH modern magneto, I fitted a moto-gadget pulse sensor to the high tension lead and it’s been working well now for over 10 years, surprisingly even with non suppressive plug caps. Recently I decided to fit a rebuilt Lucas KVF magneto with a new ATD and I clipped on the pulse sensor the tachometer fluctuates but mainly stays wound around the dial. I immediately fitted suppressor plug caps, then rechecked the setup of the tachometer, I made sure the leads are isolated from each other and removed the kill button wire, still the same problem, I’m bewildered what’s wrong anyone got a idea how to fix it please?
Misc: Electrical (General) Tachometer pulse sensor problems
- Thread starter indian-vincent
- Start date
I’ve tried that and today reversed it as well as fitting resistance plugs, plug caps and leads, the strange thing is when it had the modern B-TH I was using non resistance plug caps and copper core lead, the tachometer read normal. It must be something that’s interfering with the signal but I can’t work it out.
I think Vic might be on to something.......remember a magneto is an AC device........On race bikes using an electronic Scitsu tacho, there are models of them available that suit magneto ignition only........perhaps they have an inbuilt diode that lets the mag signal only pass one way.......just a thought........Cheers........G.
Good thought but I’m using a standard Lucas magneto now and that’s the one that’s effecting the tachometer, before used the modern magneto which gave no problem
You should try putting a diode between the moto-gadget sensor and your tachometer. The output from the sensor will be minimal so a small one is OK. ( If you don't have much electronics knowledge a diode is basically a one-way valve.) A 1N4001 diode costs literally pence. Do a search on the internet. If it doesn't work try connecting the diode the other way around as it may be blocking in the wrong direction. There were tachometers that looked like the Smiths ones available a few years ago but the one-man band stopped making them. These worked once a diode was fitted in the feed to the tacho.
Brilliant idea, I will try that thanks
Here is a long shot, try the ht lead adaptor the opposite way round. A lot of timing lights have them and lots are directional. Also most Lucas mags do not perform their best with resistor leads, caps or plugs.
A KVF Magneto is a very different beast to a BT-H. Magnetos produce a lots of electricical noise. The tachometer is picking up all that noise and thinking that the engine is doing a million RPM rather than tickover.
As others have said, don’t use resistor plugs with magnetos. They are there to suppress some noise but at the expense of wasting some of the energy. As magnetos generate less power than the BT-H, as soon as the output drops off (heat, age or just the curse of Joseph Lucas the Prince of Darkness) your sparks might stop.
Reducing the noise going into the tachometer is your challenge. Does the tachometer drop to zero when you unclip the sensory from the HT lead? If so then BigEd’s suggestion of putting the diode in might help. If it doesn’t then the noise is getting in through the other wiring to the tachometer.
Let us know how you get on!
As others have said, don’t use resistor plugs with magnetos. They are there to suppress some noise but at the expense of wasting some of the energy. As magnetos generate less power than the BT-H, as soon as the output drops off (heat, age or just the curse of Joseph Lucas the Prince of Darkness) your sparks might stop.
Reducing the noise going into the tachometer is your challenge. Does the tachometer drop to zero when you unclip the sensory from the HT lead? If so then BigEd’s suggestion of putting the diode in might help. If it doesn’t then the noise is getting in through the other wiring to the tachometer.
Let us know how you get on!
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