# Homemade CDI ignition

#### SimonTHK

##### Member
Hello electro people!

I have a jetski, and now something have burned in it twice, something that cost a student 150$each time it happens. With all the great and fantastic help, I am sure I can build this myself. But I need the help A similar jetski: 1991 KAWASAKI 650sx$1,200

The problem on my ski is that it burned the CDI ignition twice. This is kinda like a programable spark plug ignition.
Right now I look at this "guide": SportDevices. CDI Programmable Digital Ignition.
and it is the same picture I have put here with numbers, so its easy for me to ask and you to help.
It is not my idea to use the same microprocessor or follow this guide 100%, I just need to know what it is all about.
Please have in mind that I havtn studied much over a year.

Q1, number 1: Would you build such and ignition coil yourself? Or can it be bought? It has 200-300v primary and 30k v secondary.
Answear:

Q2: The guide is for one spark plug, my machine have two. I guess I could use the same ignition coil for both spark plugs, I just have to lead the current with a mosfet or what would you use?

Q3, number 6: I know that an ignition coil makes the spark plug spark when it discharges right? How does it disharge in this schematic? Through the thyristor? When RA2 (number 2) is high?

Q4: In this circuit the microprocessor should register the "pick up" pulse, have a delay time programmed and then make an output to the thyristor. Can anyone confirm this?

Q5, number 2: There are shown 2 diodes with a resistor. The right side is from the Uc to send an ignition signal right? What is the left side doing? I cant figure that out?

I have more quistions but lets start with these

#### BrownOut

##### Banned
Interesting project:

Q1, number 1: Would you build such and ignition coil yourself? Or can it be bought? It has 200-300v primary and 30k v secondary.
I doubt you have a 200-300 VAC alternator on your jetski. The TCI solution would probably be more appropriate.

Q2: The guide is for one spark plug, my machine have two. I guess I could use the same ignition coil for both spark plugs, I just have to lead the current with a mosfet or what would you use?
I don't know a thing about jetski's, but my old two cylinder outboards dont' have a distributor. Both plugs fire at the same time, but only one is on the combustion stroke at a time.

Q3, number 6: I know that an ignition coil makes the spark plug spark when it discharges right? How does it disharge in this schematic? Through the thyristor? When RA2 (number 2) is high?
The spark plug fires when the capacitor discharges through the coil primary. The curcuit is coil primary-.68uf cap-SCR-back to coil.

Q4: In this circuit the microprocessor should register the "pick up" pulse, have a delay time programmed and then make an output to the thyristor. Can anyone confirm this?
Sounds right. I didn't read the whole article.

Q5, number 2: There are shown 2 diodes with a resistor. The right side is from the Uc to send an ignition signal right? What is the left side doing? I cant figure that out?
It appears to supply the pulse to fire the SCR while the engine is cranking, and while the PIC isn't yet running. i.e. it's a wire or.

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#### KeepItSimpleStupid

##### Well-Known Member
Q1. Doubt you could build a coil. Flash trigger transformers may be suitable. Car ignition? Don't know.

Q2. I think you need a minimum of 4 cylinders. The extra one has to fire in the exhaust stroke. Intake compression power exhaust.
With 4 you can find another with compression power exhaust intake order.

Q3. cap discharge. You discharge the cap into the coil.

Q4. Depends on RPM. In one case your firing before the trigger. Its in the documentation.

Q5. have to look on a bigger screen.

#### BrownOut

##### Banned
Q2. I think you need a minimum of 4 cylinders. The extra one has to fire in the exhaust stroke. Intake compression power exhaust.
With 4 you can find another with compression power exhaust intake order.
Say... what? He only has two cylinders to start with.

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#### KeepItSimpleStupid

##### Well-Known Member
Right. A condition would be one cylinder is exhausting while the other is in the power stroke. I said I doubt that was possible with 2 cylinders. With 4 cylinders it definately is possible.

#### BrownOut

##### Banned
It's totally possible with two cylinders.

#### jpanhalt

##### Well-Known Member
Modelers have been active in homebuilt CDI and TCI for some time. Here are three forums/threads with useful information:

DIY Electronics - RC Groups
RE: Opensource CDI ignition
RE: New CDI - opensource project JMJ and Bigboat

There are more commercial/finished sites:

SportDevices. CDI Programmable Digital Ignition.
Silicon Chip Online - Replacement CDI Module For Small Petrol Motors
becker-fm.de

Today, the cheap Chinese imports that work seem to have blunted interest in DIY projects.

Just for reference, I have attached some reverse-engineered CDI schematics.

John

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#### BrownOut

##### Banned
Ah-ha! At first I wondered what the PIC was for, but now I see it's for advancing the spark. BTW,

jpanhalt said:
Today, the cheap Chinese imports that work seem to have blunted interest in DIY projects.
Isn't that true of almost everthing?

#### crutschow

##### Well-Known Member
It's totally possible with two cylinders.
Yes. Vertical twin engines with a 360 (0) crank or horizontally opposed twins with a 180 crank can operate that way.

Edit: This applies to 4-stroke engines, not 2-stroke.

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#### BrownOut

##### Banned
After reading KISS's post more carefully, I think he's saying because the opporsite cyclinder is on the intake stroke, it cannot be fired. It works on my old outboard motors because they are 2-stroke. Possibly that's the case on the old jetski as well.

#### SimonTHK

##### Member
thanks to you all. Ill look more into all of this and come back with alot more quistions

#### KeepItSimpleStupid

##### Well-Known Member
Yep, your analysis is correct.

I can see 2-stroke working.

This is a fascinating subject. I did convert a 1965 Ford to an opto ignition in 1973 and got great milage. the spark plug cables had to be upgraded too and that was difficult.

a) i didn't see anything mentioned about a pickup source.
b) I'd probably change the 1n4007 at the coil to a faster TVS type diode if possible.

#### Jaguarjoe

##### Member
Motorcycle engines should be pretty similar to a jet ski engine. They would be a good source for coils and things.

This whole thing is pretty much dead in the water without some kind of spark advance map to squirt into the PIC. Just guessing at this could hurt the engine.

#### KeepItSimpleStupid

##### Well-Known Member
From the website
"First graph shows the real advance curve measured in degrees over the PMS, nevertheless the PIC can not perform advancing, else it will make a delay from the pickup signal (typically at 36 degrees). For the maximum advance point, the PIC will do a zero delay (for 36 deg. advance). When this maximum value is determined, the second graph can be done, this graph shows delay in degrees that the PIC should perform after receiving the pickup pulse.

The PIC computes the elpased time between the last pulse and the present one, and by using this count it access to a table in which are stored time to delays vs measured period."

I think it has to do with interpretation. Timing advance is being done, but not with respect to the sensor. An "advance" means delaying the firing from the trigger a "long time". The statement, I think is poorly worded. The amount of delay is based on RPM. Asa dumb example say it makes 1 rev/60 sec. 1 RPM, impossible OK.

Trigger. Delay 5/60 of a sec delays the spark. Remember that there is also some reaction time as well, so 5/60 of a sec could even be TDC.

If you delay 56/60 of a sec, you have advanced the timing.

The determination, is based on a look up table based on RPM.

Low RPM is ignored and the plug fires based on the pickup.

#### BrownOut

##### Banned
You're damn right it was poorly worded. Thanks for helping clear it up.

#### crutschow

##### Well-Known Member
KISS is not following his motto and is making it more complicated then it is.

The PIC obviously can not advance the spark, only delay it. So the pickoff signal is set at the maximum advance point (36 degrees before TDC for the engine shown). The PIC generates maximum delay of the spark (equal to about 36 degrees) at low speed, giving little or no advance from TDC, with the delay inversely proportional to engine speed. Thus as the speed increases the delay is reduced, advancing the spark, and at high speed the is no delay, giving the maximum 36 degree advance.

#### KeepItSimpleStupid

##### Well-Known Member
Darn. You did a better job.

OK, what sensor might be used?

#### SimonTHK

##### Member
I guess it uses ignition coils as sensors, if you mean for the pickup signal.
So I mean the induction from them

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#### jpanhalt

##### Well-Known Member
The model CDI's I listed typically use Hall sensors today. Some older units, for example, the 3W units form Germany, used coil sensors. The triggering of the CDI is a little different for coil vs. Hall sensors.

I don't recall you saying whether your motor is 2 cycle or 4 cycle? How are the cylinders arranged, e.g., in-line, opposed, or V? Firing order?

John

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#### buju357

##### New Member
I have worked extensively with 2 stoke engines.
all 2 stoke twins fire at the same time , when one piston is on the compression and the other on the exhaust. they have 2 pickups 180 degrees opposed on the stator housing. 1 coil splits into 2 leads , each going to a cylinder. The reason they program cdi's is to get better low and top end power by varying the timing. effectively if your 2 stoke revs to say 8000 rpm its going to be sparking at 8000 x 2 = 16000 rpm.
a small car coil WILL work , its one of the tricks to get a higher voltage spark at the pliugs or if the original coil burns out.

btw - I have a 1982 yamaha RD 350 LC with 45k km's on the clock and the coil is still working , coils seldom burn out.