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I want to be able to delay the signal that goes from the cdi box to the igniton coil on my moped.

I would like to be able to control the delay with something simple like a potentiometer, i don't need to be able to know how much the signal is delayed.

Besides this, the device should have some switch to turn it on or off with so that I can set the delay once and then not touch the potentiometer or whatever it may be, just the switch that controls if the signal is retarded or not.

What's the easiest way to do this?

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – Dave Tweed Jun 24 '18 at 11:57
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Unfortunately, this is all but simple. Moped CDI boxes are extremely optimized on parts count for reliablity purposes. It's usually nothing more than an extra coil on the alternator for generating the trigger pulse, a fat thyristor, a capacitor and a resistor. And the spark plug coil of course.

Meddling with it means redesigning the whole CDI box.

BUY ONE with the properties you need. They are available below $100.

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  • \$\begingroup\$ Thank you but this isn't the answer I was looking for :D \$\endgroup\$ – Kalle Hiltunen Jun 23 '18 at 21:27
  • \$\begingroup\$ … But it's the answer you need! \$\endgroup\$ – Marcus Müller Jun 24 '18 at 8:03
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Trying to delay the output of the CDI box is going to be a non starter. What you want to do instead is to apply a delay in the path of the input trigger pulse to the CDI. This will be an essentially digital trigger type signal that should be easy to delay.

You will not be able to delay the trigger signal by fixed amount however. The amount you need to adjust the "delay" will depend upon the RPM of the motor itself. So what you really probably need to do is to deploy a microprocessor into the design that constantly monitors the time from trigger pulse to trigger pulse. A timer module on the MCU and some simple software can get that job done. Then you can monitor your POT using an A/D input to the MCU. This measurement will then be used to scale the time between the incoming trigger pulses to come up with a time delay to apply to the delayed trigger.

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  • \$\begingroup\$ Allright. I don't need the engine to run smooth, just make it rev lower and kick out less power, would it really not be possible to delay the signal for a fixed amount of time everywhere on the rev range? \$\endgroup\$ – Kalle Hiltunen Jun 24 '18 at 5:47
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    \$\begingroup\$ No it would not work with a fixed delay for every engine rotary speed. Lets look at it with some numbers. For example if you wanted to delay the CDI output by say four degrees from top dead center (TDC) at 3000 RPM that time might be 1.11msec for a four stroke engine. If you halved the RPM the time to trigger the CDI at the same crank shaft angle would double to 2.22msec. If you followed that a fixed delay of 1.11msec at the 3000 RPM would fire at four degrees whilst at 1500 RPM it would be firing at 2 degrees of crank shaft angle. \$\endgroup\$ – Michael Karas Jun 24 '18 at 9:39
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There is no circuit that will introduce delay to an arbitrary electric signal except a really long wire (and even then, only if impedances are matched). This is just not an easy thing to do. You can, however, design circuits that will delay some kinds of signals, under some circumstances, OR make a circuit that will produce a second signal a while after the first one triggers it.

In the case of a CDI box, the signal that activates the ignition coil is really awkward to delay without essentially making a new CDI box. In that case, it's best if you get its specs.

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  • \$\begingroup\$ Something mimicking the original signal would work too. What do you need to know about the cdi box? \$\endgroup\$ – Kalle Hiltunen Jun 24 '18 at 5:43
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    \$\begingroup\$ There was a circuit to introduce delay within ancient german PAL color TV receivers, an ultrasonic delay line. There are electronic delay lines made from a chain of serial inductors and parallel capacitors. See Wikipedia about analog delay lines. So there are other circuits besides the very long wire. \$\endgroup\$ – Uwe Jun 24 '18 at 17:01
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The easiest way to delay the ignition is to move the crank position sensor. Perhaps you could mount a second sensor and use a switch to select which sensor the CDI sees.

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