Discussing CDI charateristics - Page 3 - Kawasaki KLR 650 Forum
1987 to 2007 Wrenching & Mods For maintaining, repair or modifications of Generation 1 KLR's. 2007 and earlier.

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post #21 of 23 Old 01-16-2019, 04:44 PM Thread Starter
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Sure. I'm using a Pico Technologies two-channel USB scope.

The blue trace, of trapezoidal shape, is the signal from the pick-up coil. That is the coil that is adjacent to the rotor and which reads the reluctor bar. It creates a signal that it sends to the CDI, telling it to dump the energy in the capacitor. That, through the ignition coil, creates the spark at the spark plug.

The red trace is being picked up at the tach signal off of the ignition coil. It represents the ignition spark.

I recorded both of these traces earlier on my single-channel pocket scope, seen a few posts above. I did that to see what they look like and what their relative magnitudes are. The pocket scope is good for quick and dirty stuff as it is so easy to fool with, but there is only one channel. With the rain, I've got a good excuse to waste time and not do productive work like picking weeds and mowing the lawn, so I've gotten around to building some test cables to grab signal with and hooking up the Pico.

What we're seeing is the spark, which is supposed to be at 10*BTDC at idle, advancing to what is supposed to be 40*BTDC as I increase the engine speed up to about 4000rpm. It's hard to figure out what the advance curve is from this video because the time scale is in milliseconds and the number of milliseconds that corresponds to a certain number of degrees of crank rotation changes as the rpm increases.

In the first two videos, I had the scope trigger set on the blue trace, trying to set it just before the peak of the trapezoid. I wanted to measure the spark advance from there. The ignition was putting noise on the blue trace and carrying the trigger point along with the red trace, so the output jumped when the noise on the blue line decayed away enough. I changed the trigger to the ignition, which has a negative spike I could rely on (the blue trace is all positive), so we watch the blue trace retard away from the red trace instead of the red trace advancing away from the blue trace. Mathematically the same.

I have taken some readings at idle, 1000, 1500, 2000, 2500, 3000, 3500, and 4000 rpm to try and figure out what the advance curve is. The scope has rulers that can read the output in degrees, but it's a PITA. I have numbers to crunch.

By the way, @pdwestman, I have tried to see the effect of the Y/R wire, energized and not. The most I could see was that it made the spark signal slightly jittery. I could not see any spark retard. I'll have to take the time scale down into the microseconds ranges to see if there is any retard. I still think it just defeats an anti-kickback function.

Tom [email protected]

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post #22 of 23 Old 01-16-2019, 06:55 PM Thread Starter
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Variables are my ability to capture traces at the specified RPM and my ability to interpolate the scope timescale. The curve is pretty linear, though, so it seems like I did OK.

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Tom [email protected]

“If I wasn't hard, I wouldn't be alive. If I couldn't ever be gentle, I wouldn't deserve to be alive.” -Philip Marlowe

“'Why' and 'How' are words so important they cannot be too often used.” -Napoleon Bonaparte


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post #23 of 23 Old 01-18-2019, 10:56 AM
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Great work!

What is the operational mechanism for the advance?

As the field hand once asked (amazed when a shooting party guest poured hot coffee from a container on a cold winter day, informing the worker his thermos flask kept hot liquids hot, and cold liquids cold), "How do it KNOW?"


Last edited by Damocles; 01-18-2019 at 11:04 AM.
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