CDI Information, Good Stuff

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2door

2door

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I posted this link in another thread but thought it might get more exposure here.
There is some good info here and might answer some questions about the ignition system we all deal with on the Chinese 2 stroke engines. Be sure to click on the McLaren Electronics link at the bottomn of the page in the 'References' area for some detailed text on the CDI and how it works.

Capacitor discharge ignition - Wikipedia, the free encyclopedia

Tom
 
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matt167

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so it's a system that does use an inductive coil, but uses a capictor to store engrgy and electronics to determin the correct time.. really, doing the same thing as a set of points and condensor or magnetic trigger, but without any moving parts.. with this setup tho, that means there is a degree of ignition 'timing' done with the capacitor system.. if someone could find which resistor value to change to increase or decrease timing and a way to open them up and seal them back.. a variable resistor could be used in place and adjust the timing, which could result in better power
 
2door

2door

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Matt,
A couple of us have been discussing a point in PM and I'd like your input on this: In your opinion what importance is the magnet (rotor) position in relation to the coil? Here's some info for you. I've checked 3 engines, all identical, same bore/stroke, same carb, CDI, plug and plug wire. One is a dog, one runs good and one is a powerhouse. There is a noticable difference in power between the best and worst. I made an adapter that would allow me to mount a cam degree wheel to the crankshaft and used one of the clutch actuator cover screws as an index point. With the pistons at TDC the best engine is 2 degrees advanced over the one with the least power and 1 degree advanced over the middle engine. When I say advanced, I mean the keyway that indexes the rotor and therefore the center of the magnet in relation to the coil. I don't know what importance that is except that it would appear that having the magnet closer to the coil center influences engine power. Or at least it appears that way. The 'ignition timing' is the only measurable difference between these three engines so it suggests to me that it is a significant factor. Could the rotor position in relation to the coil allow for a shorter or longer saturation time? I'd like to hear your views on this. Norman and Jim, chime in here if you would.
Tom
 
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matt167

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I don't think it has much to do with saturation, as the CDI would eliminate all effects of that, because it does use induction and a coil inside of it.. it could saturate more or less, but those effects wouldn't be seen.. the timing would be what is making the diffrence.. 2* advanced makes sense that it would make more low end power.. if you found there 3* in varation on keyway position, it would appear they are keyed as 'close enough' rather than be precisely machined...

another option is to take the mag off and have a machine shop set it up on a degree wheel and machine a new keyway... perhaps someone could machine them and have them indexed for 2* and 4* advanced as well as 0* straight up, and sell them.. of course, advancing too far will increase cylinder heat and could run into detonation and burning a pistion up, which is even more of a problem on an air cooled 2 stroke.. so engines that have too much timing advance would need to be mod'd with more fuel and a colder plug
 
2door

2door

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Matt,
I couldn't agree more with you regarding the 'too far advanced' problem. That is a fine line that can be delt with in race car motors with altering spring weights on the distributor rotor, different vacuum advance curves and such but there would be no way to control ignition timing based on RPM for these little motors without some very fancy parts and pieces. You mentioned the low end torque being better but the more powerful of the 3 motors I've tested has more power across the band, not just down low. There almost has to be a correlation between the magnet position at or near TDC but without a complete understanding of the way the CDI is triggered I can only guess at what that might be. You might be right about the manufacturers just getting the keyway 'in the ball park, but that also might explain why so many guys have experienced what I have with having good and poor, running, yet identical engines.

Junster,
Have you checked to see if the offset keys will fit the crank/ magnet of a Chinese 2 stroke? I know they make them for automotive use, I've used them, but wasn't aware they had offset woodruff keys for scooters and such. What range of advance/retard do they offer? Thanks for the input.
Tom
 
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matt167

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right, there is no timing advance on them. changing the keyway position would change the base timing and therefor, the total timing.. if there is any timing advance, it would be done electronically in the CDI probably based on mag voltage, but I don't know any way to check to see if there is, but it's doubtful that there is any... thinking about it, the magneto could change the timing based on what point it really charged the wires and at what frequency, because that would corrolate to what the CDI did.. on these 2 strokes, advancing the timing may have more of an effect on a 2 stroke, because they fire every cycle
 
Junster

Junster

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This is what they look like. If you do a search alot of places have different ones. If you just get a woodruff key with the right round side but too thick it's pretty easy to file the offsets in one. (and alot cheaper)

Honda CRF50 Timing Adjustment Key 2*° 51-253200 - ADA Racing

PS. You can also file the keyway in the magnet wider and use a piece of copper house wire to fill the gap. You just hammer it flat till it's the right thickness to slip in next to the woodruff key. That's how they hop up chainsaws.
 
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gian

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is it the cdi that gives timing to the sparkplug .i dont know if im wrong but isnt the magneto that gives the timing everytime the magnet rotates it fires up and it depends on how fast the rotation of the magnet . the speed of the magnet it same speed as it fires up. cdi just act like a step-up transformer i dont know if im right but does my own thoughts
 
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noco

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fort collins colorado
hey finally...ive been trying to talk about this for months...i was looking at 2 stroke scooter parts trying to determine if the trigger voltage from the magnito would cook it...cant find any specs...i only got a graphing multimeter and i dont want to chance cooking that to measure the output voltage of the magnito...does anybody have any ideas for this???i have a 4stroke scooter but i cannot measure what comes out of that cranksensor due to thats the only broken part on it!!!the scooter has a much smaller reluctance generator to supply the signal...maybe if the correct angle to mount a crank sensor to replace the magnito with some form of a reluctance generator (crank, cam, wheel speed, turbine speed, etc....)then a scooter cdi might work....these 50cc motors in the scooters blow our 66cc motors away....the scooters arent as cool...
 
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noco

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too late to edit...one more thing would need to change...the magnito rotor...how about a round aluminum one with a inlayed piece of steel...would make it easier to time as well...
 
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