Explain electric ignition to me...


Active Member
Local time
7:55 PM
Mar 19, 2008
South Mississippi
I think I have a magneto. Everybody told me when I wanted to wire in a toggle on-off switch (instead of the grounding out method), that I would need to splice the switch in between the magneto and CDI. I don't see a CDI... is that possible, or is it all built into one little box?

The box has two wires that were originally for the momentary switch (to ground out), black & red... and there's also the spark plug wire. At the bottom is the magneto. All in one, right? The thing's glued up pretty nicely.

Also... The plug connector is soo poor quality, so I was gonna try splicing a spare plug wire this guy gave me, and when I cut into it... I didn't find ANY wire. Just some rubber, fibers, & plastic. I really must be losing my marbles.
I guess the best thing for me to do is try and find a Tanaka magneto/CDI, since this is supposedly a rip-off of the Tanaka 33cc.
There really is nothing wrong with grounding (shorting) the magneto. It produces bugger-all current, and has a pretty high resistance. You would be dissipating the energy across a LOT of wire, it wouldn't even get warm.

Spark plug wire used on these (as with most (all?)) engines is resistance wire to suppress EMF. It is carbon fibres + graphite IIRC. Apparently it unscrews from the CDI.

A non-resistance wire will produce a hotter spark, but will basically swamp any am radio reciever (FM will be pretty bad) in a 15m radius. Not very sociable.
Well, my bike came with stranded copper wire, and the plug wire from some guy's car had NO wire whatsoever! I couldn't understand that.

Anyway, I'm guessing my magneto and CDI are both in this little black box, I couldn't just have a magneto, right?

Hopefully just temporarily, I've put the magneto/CDI/plug wire from my old engine on, and it runs alright... BUTTT... it whistles now, like an alien ship floating right above/behind me. At least, I'm pretty sure that's what it is. I'll have to take the cover off again to take a closer looksy & listen.

I shoulda just bought a Tanaka.
Magneto is inside the engine - there is a magnet on the flywheel/crankshaft, and the coil is fixed to the body of the engine. The black box is the CDI.
Yea, my magneto & CDI are an all-in-one unit, I guess.

I guess I should look into seeing if a genuine Tanaka replacement will actually fit this Tanaka knock-off.

EDIT: wait a minute, are the magnets on the flywheel considered the magneto... or is the "pickup" magnet with the coil the magneto?? And is there even a capacitor in this "CDI", or are there just one or two coils?

I also looked at the Tanaka replacement "coil, ignition", and it seems that one of the bolt holes is in a different place, so I'm stuck with dealing with FiveFlags. =-(
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There are magnetos that directly fire a plug,without any CDI unit in fact it is incorrect to talk about a magneto with CDI.The original magnetos were originally developed by Robert Bosch in Germany and directly generate a spark.They had big horseshoe magnets on them hence the name "magneto".These magneto based systems were standard for many years.Later with batteries becoming more common,battery ignition gained a foothold (cost).Until about 20 years ago all magneto ignitions had breaker points (contacts).The spark discharge would be initiated when the points opened and the energy stored in the magneto HV coil is released in a high voltage pulse (actually a series of pulses) to fire the plug.More recent magneto systems use a solid state switch instead of the "points",but the principle of operation is the same.
Capacitor discharge systems use the stored energy in a capacitor to fire the plug (via a pulse transfomer).They can generate very high voltage, but shorter duration, high energy ignition pulses that can fire even leaky plugs,but may not be as effective in firing the micture due to the short pulse duration.Most car ignitions still use coil discharge systems, which are just as effective.Sophisticated CDI systems are widely used for racing applications, they generate a series of high energy pulses for fast burning of the micture charge
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I'm confused too... I researched electronic ignition and it seems to require a battery, but these engines dont have a battery...
I might be able to help offer some assistance on how these systems work. I don't want to be real technical, but if you know a few things; these circuits are pretty simple.

oh @Sparky, the squeak noise is a "leaking" HV cap in the CDI (black box) prolly should replace it.

Okay onto the ignition system.

-First you have the magneto (magnets on a rotor). With these systems it serves 2 purposes; Both timing and ignition energy. Timing is, of course, "where" they are placed in relation to the pistons' degree of cycle. Ignition energy comes from both the strength of the magnets (mainly) and the speed they are moving.

-Second you have a coil (essentially a "pickup" like in car dizzy's) This receives the magnetic energy (Iron core) and transfers it to the CDI through the "primary" wires wrapped on the Iron core.

-Third you have the CDI circuit. This "stores" the energy in capacitors (condensor in a car) and then transfers this energy BACK through your coil (in a different wire).

-Fourth, the coil (secondary circuit) "steps up" or transforms the "stored" capacitor energy to High Voltage, this goes through the spark plug wire.

That's basically the operation step by step. So, to directly answer the question "why no battery" the "juice" is coming from the magnets alone.

A couple of other things that may be worthwhile to point out as well.

Something key if your playing with timing in these systems. The HV spark "fires" just AFTER (ms) the magnet (s) pass the coil. I can explain more about this, but I don't imagine it's necessary. So the "trailing edge" of the magnets is closer to the firing point in time. Don't time off the front or middle.

Ignition energy can be affected by Magnet strength and spacing to the "Iron" part of the Coil's core. With Magnet's too weak or spacing too far; not enough energy gets transferred and stored to create a HV spark.

For Coils (in these low energy systems) they would probably fail slowly and you would get a weak spark. You could also test the wires if you can access them.

For the CDI's they should be mostly a "go no go" situation as their parts usually fail catastrophically. You have the one exception with a leaky cap and a squeak. ;)

I think that covers it, lol. Sorry to be windy. Hope this helps
Wow... Didn't even know I had started such a thread YEARS ago.

If I use a voltmeter to test the ignition coil, should I just take the spark plug out.... and put one wire on the ground (engine head) and one on the plug's inner electrode dealy??

I shipped my two faulty ignition coils back to the dealer for him to test at the price of $6, not even thinking about harbor freight selling voltmeters for less than $10....