Topic: Question about CDI in old bikes

[Raul CB750K1]

The story goes like this: you know I'm restoring and old spanish Sanglas bike. I will be using Honda switches and some other things to make it more "rider-friendly".

That bike was known to be a "leg-breaker", seriously. Just a few months ago a guy from the club got his tibia broken. The kickstart pedal is directly geared to the crankshaft. If you have a too advanced ignition, when trying to kickstart the bike you can get a violent hit by the returning kickstart pedal.

As I said, I want to make it some more "rider-friendly". I was planning to use an electronic ignition "a la Hondaman", to reduce the current through the points. But then an idea crossed my mind.

I have some spare CDI's from a modern Suzuki GZ125, along with the schematic. The CDI determines the moment of ignition by means of a pickup coil. The ignition advance is calculated by the electronic device, not necessary to use the advance unit. Due to the principle of the CDI, the coil is smaller -I have spare coils too-.


That's when the question arise. By trial-and-error I'm positive I can set the static timing. But, what about the dynamic timing? From what I understand, the advance ignition is necessary because the mixture doesn't explode when the spark ignites, but some milliseconds later. If that's a chemical thing, it should be independent from the bike make or model.


That is, the CDI, by counting the coil impulses by time unit, calculates the engine RPM. The CDI calculator knows how many degrees is the coil with regard of the TDC. Knowing the rotational speed, and the relative position of coil and TDC, the CDI calculator knows when to ignite the spark. From my understanding, this should work in ANY bike -a thumper for that matter-. Am I wrong? Does the angle between TDC and FIRE change with different bikes?

[gerhed]

Raul,

I have CDI on an '83 Honda XL600 single (it came that way).
The kickstart mechanism makes a "click" when you reach a point near TDC.
Has an automatic compression release at that point, too.
Then kick as hard as you can and it usually starts.
When starting a hot engine, however, all bets are off.
Afraid that's the extent of my CDI knowledge.

gerhed

[Raul CB750K1]

Thanks for your replies. The CDI unit is from a single, and it would go on a Sanglas single. Obviously, I would get rid of the mechanical advance unit, install a metal bracket on the crankshaft end -with a metal prong-, and then a baseplate where the coil pickup would be installed, in such a way that there would be an impulse everytime the metal prong crosses in front of the pickup. I know from the GZ125 shop manual where the pickup is with regard to TDC, so I can get pretty close on the Sanglas and make slots on the baseplate for adjustment, like you would do on any electronic ignition.

Some food for thought. DG, you say that the advance depends on the lenght of the stroke. Obviously, if the piston goes from bottom to top with half a turn of the crankshaft, it makes (stroke) with 180º. The longer the stroke, the longer the distance-per-angle even when it is not linear -it advances faster at 90º and slower at 0º and 180º, if you can see the image of the piston going from botton to top, being bottom 0º, 90º when the crank arm is parallel to ground, and 180º when the piston is at TDC.


BUT, say you have two engines with different stroke lenghts, both running at the same RPM. As we are talking now angular rotation, not linear movement, it should take the same time for both engines to turn 5º, 6º or 40º. As the advance timing depends on time -advanced time you need to spark so the explosion occurs at TDC-, it should happen at the same angle from TDC irrespective of the stroke lenght, or at least that's how I see it. Isn't it?

[Raul CB750K1]

I understand about the different linear speed. From my understanding, if it takes -say- 2 milliseconds for the mixture to explode, the spark has to happen 2 milliseconds before the piston is at TDC. If both engines are turning at the same RPM, they turn the same number of degrees each 2 milliseconds, so in both the spark should happen at the same number of degrees before TDC. The different strokes would mean that one of the pistons will be closer (in distance) to TDC than the other, but when both crankshafts turn those given degrees, both pistons would be at TDC.


This is the way I see the ignition principle and that's why I am asking iwhether I'm wrong or right, because if I'm wrong, I will not be able to use a CDI from another bike, but if I'm right, it means that the angle of advance is the same irrespective of stroke or bore, so any CDI could be used for another bike as long as the pickup coil is installed at the same angle than in the original bike.