How A CDI Works (Gen 1) - 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 #1 of 34 Old 02-26-2014, 05:23 PM Thread Starter
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Arrow How A CDI Works (Gen 1)

"What you always wanted to know about your CDI, but were afraid to ask!"

I share this YouTube video clip with you, fellow Generation 1 fans, because . . . it is VALID!!!!!!!!!!

In my limited research, I found several YouTube video clips PURPORTING to explain CDIs; however, I found them quite erroneous/incomplete/misleading/etc.

This one, though simplified, you can generally take to the bank, operational principle-wise:

CAVEAT: Remember, I said, "Generation 1." Generation 2s have no CDI (Capacitive Discharge Ignition) whatsoever.

Enjoy the show!
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Last edited by Tom Schmitz; 02-26-2014 at 06:19 PM.
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post #2 of 34 Old 02-26-2014, 05:37 PM
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Awesome thank you. This is also the basic principles for kick start magneto driven bike, boat outboards and airplanes.......nice add....sticky please!

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post #3 of 34 Old 02-26-2014, 06:10 PM Thread Starter
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Glad you like it, Tractorking!

In consideration of the unfortunate riders of Generation 2 KLRs ( ), I posted a link to a video clip illustrating transistorized inductive discharge ignitions on the Generation 2 forum.
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post #4 of 34 Old 02-27-2014, 12:23 PM Thread Starter
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Here's some verbiage extracted from a comment I made on the Generation 2 thread:
I mentioned the Generation 1 presentation was simplified; refinements I'd expect in the real-world as-built condition: First, the AC coming from the exciter coils could easily be stepped-up with a transformer; I expect the elves who assembled Generation 1 KLR650s performed that very trick--say, going from maybe 100 VAC to 400 VAC. Next, the schematic shows a half-wave rectifier (the diode, D1?); no reason not to put in a full-wave rectifier, increasing the capacitor-charging coulombs-per-revolution by about 100 % (i.e., nearly twice that of a half-wave rectifier). So, by these refinements, the voltage is increased by a factor of 4, and the charging time and thus, current, by 2.
Disclaimer: Don't know that these refinements actually exist in the Generation 1 CDI; speculation only.
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post #5 of 34 Old 09-16-2014, 04:12 PM
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Finally looked at the thread. Has anyone de-potted...un-potted...reverse-potted a Gen1 CDI?

I'd expect it has a more sophisticated circuit than the one illustrated, including advances such as mentioned. It will almost certainly have some means of compensating for temperature effects on switching, etc.

Has anyone run the KLR CDI on a test bench or 'scoped to see whether the ignition advance is processor based within the CDI box or due to stepped trigger coil core?

It's been on my list of things to do but don't seem to get there.

Here's a "make your own" CDI for small engine which explains some mpre advanced features often found in commercial CDI. The stepped core for trigger coil explanation is what I referenced higher up. Interesting and "simple" concept. It would seem that an advance "curve" rather than linear advance could be possible with such a design.

This also illustrates the hard to convey concept of the effects of trigger coil air gap on timing and timing advance.

Last edited by Normk; 09-16-2014 at 07:33 PM.
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post #6 of 34 Old 09-16-2014, 07:11 PM Thread Starter
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Here's a "make your own" CDI for small engine which explains some mpre advanced features often found in commercial CDI.
WHERE, Normk?

Saw no image nor link in your post.

Good point. KLR650 hop-up aficionados have fooled with trick valve timing, etc., allegedly gaining horsepower, but . . . WHO HAS EVER TRIED OPTIMIZING THE IGNITION ADVANCE CURVE MAP?????????????

WHO HAS EVER EVEN ADVANCED SPARK TIMING? (May need high-octane fuel, but . . . there could be some extra power there.)

(I'm not even going to bring up INCREASING COMPRESSION RATIO!!!!! )
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post #7 of 34 Old 09-16-2014, 07:58 PM
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Someone was posting regarding changes to advance curve and one could likely achieve some success through monitoring intake vacuum at a given throttle opening. Problem is, the KLR carb has no easy means to attach a TPS (throttle position sensor) and single cylinder engines are a real tough call for MAP sensors.

When determining ignition advance curve from scratch, the only way I know to do so is to monitor for detonation while experimenting/monitoring ignition timing at wide open throttle. Once one as determined maximum advance during the progression from minimum to maximum RPM at wide open throttle and assumed maximum temperature, one has a basis for timing the engine. This is generally where bikes stop.

Assuming the same cylinder filling, combustion rate/burning rate requires a certain amount of time. Let's call it "X" miliseconds. If the spark ignites the air-fuel mixture at a piston position which allows "X" milliseconds of burning time before the piston reaches the top, then double the RPM and the piston will reach top before the burning is completed if the spark occurs at "X" milliseconds, correct?

For this reason, the spark must occur at "X" milliseconds and not at a specific piston position, correct? So, since the spark is timed from crankshaft (or camshaft = same effect) position, it must be fired earlier in terms of crankshaft (piston) position as RPM increases in order to allow "X" milliseconds for burning. So, we must advance the timing (spark firing point) as RPM increases. Note that I have taken license in the explanation but the trend is correct. Note also that the amount needed to time correctly is no linear.

Since combustion rate/burning rate is at maximum at WOT and high temperature, one can assume that the burning rate will be slower during smaller throttle openings because smaller throttle openings provide less compression pressure and so slower burning rate. For this reason, partial throttle requires earlier spark firing in order to provide for the "X" milliseconds + added time.

In order to do this, one requires either a reference of engine load, usually by intake manifold vacuum or air flow rate compared to engine characteristics.

If one inspects a conventional automotive distributor, one will find centrifugal weights which provide speed related advance and a vacuum chamber which provides for greater advance as engine load is decreased.

How to do this for a KLR? Well, as above generally but in the final analysis it's far more difficult. Far easier to combine with fuel injection and therefore use the same air flow and engine speed data combined with experimentally supplied look-up pages.

I've worked with this problem on multi-cylinder engines which is doable. As for a single like the KLR with huge intake pressure pulses....there lies madness. :-)
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post #8 of 34 Old 09-16-2014, 08:05 PM
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The CVK40 used on the Vulcans had a TPS. All we need is a cheap and ready source of CVK40 pumper carbs...

The FI solution seems to me to be the most effective, as Norm states. There has been some success with a MicroSquirt unit.

The problem with those threads is that they usually go up to the point of final installation and tuning and then the poster disappears. Long term data is lacking, and the MPG and dyno results I read about were quite disappointing (though all of that may have been on the tuner).


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post #9 of 34 Old 09-16-2014, 09:06 PM
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It might be instructive to understand that a typical V8 from days past would add about 15 degrees of advance during cruise over what there would be at the same RPM and wide open throttle. On the road, one could play around to find whether the engine might tolerate more advance at some (or all) engine speeds and wide open throttle.

Clearly, there would be some fuel mileage advantage to be gained by providing a "vacuum advance" but doing so is another bucket of worms.

This might be "fun" in the same was as is having a prostate exam due to the KLR's trigger coil location and that one would require some means to modify the RPM to advance relationship. This is really easy in the minds of those of us who lack the electronics capacity to design such a circuit but doable. Problem is that it doesn't simply require an increased advance uniformly applied through the RPM range. There are units (or were) on the market which would allow a uniform advance of the entire advance curve. Problem with that, is that, one will experience one point at which advance is too great while others would benefit from more advance.

For this reason, a processor is required which will consult pages of data to compare engine speed and to output the desired advance. Such a system might incorporate intake and engine coolant temperatures to further refine because hotter conditions speed combustion and so tolerate less advance.

Then the engine load question which is added to the look up pages for modern fuel and ignition systems.

In the old days, we would add a detonation sensor and processor such as the one made by MSW. This worked well because we'd shove the timing up to the point where one would experience detonation ...just... then some more. The detonation sensor would detect combustion explosions then retard ignition timing. This worked quite well for many V8's because the detonation sensor and processor had enough sensitivity and range.

One can still test detonation sensors by idling the engine, and using a 10 mm combination wrench to tap the sensor or close by which will cause the sensor to output and the timing to be retarded. Think of the chances that an engine as rattly as the KLR's will be the ideal partner for a detonation sensor...

I've also avidly followed a number of EFI or other processor projects which have disappeared. My conclusion has been that things weren't working out and were finally dropped. Not many will come back onto a thread after a big build-up to admit failure. Especially as there will be so many people who have never touched any of this stuff who will know exactly what should have been done. :-)

I'm still interested to hear whether anyone has determined whether and how much advance is built into the CDI unit or whether some or all the advance is due to a stepped triggering.
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post #10 of 34 Old 09-16-2014, 11:42 PM Thread Starter
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Originally Posted by Normk View Post

This also illustrates the hard to convey concept of the effects of trigger coil air gap on timing and timing advance.
Thanks for the link, Normk; however . . . I missed the trigger coil air gap effect discussion in my first reading; gotta go back and read it again!

For programming the ignition map, looks like we need to take a look at this article:

To replace one of these units, you could adapt one of our previous designs, such as the High Energy Ignition (SILICON CHIP December 1995 and January 2006) or the Multi-Spark CDI (September 1997). Alternatively, you could consider using the Programmable Ignition System from March, April & May 2007.
The spark advance of the CDI on Kawasaki V-Twin ATVs (Prairie, Brute Force, etc.) is determined totally by engine rpm input (trigger coil impulse rate), electronically, without manifold vacuum input.

Aftermarket, wire-on modules ADVANCE the initial spark timing and allow the engine to stand the quads up on their hind legs (the factory configuration retarded the spark to prevent end-overs by enthusiastic but unfamiliar/unskilled riders, hoping to reduce vulnerability to product liability claims).

So . . . no reason NOT to incorporate manifold vacuum as an ignition timing input parameter (along with rpm and perhaps throttle position, etc.), but . . . not typically done with motorcycle/ATV CDIs today, AFAIK. Spark timing is varied electronically with input only from trigger coil pulse frequency, seems to me.

Gotta find that, "Programmable Ignition System" article from March, April & May 2007!
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