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Last Labor Day weekend, my brother and I began a discussion about why Vespa and Lambrettas seemed to be the only vehicles not equipped with variable ignition timing.

My brother then set forth in designing a system that could be placed betweent the pick-up on the stator plate and the CDI unit that could either advance or retard the ignition depending on the engine RPM.

The idea is that a specialized circuit could hold the impulse from the pick-up and release it to the coil at exactly the optimum time for ignition. This timing changes depending on the RPM of the motor so a digital system seemed like the best idea.

Why vary the timing of a running engine?

The gasoline/air mixture is ignited a certain fixed degrees before TDC in a typical scooter engine. The burning of this mixture does not take place instantly, but actually takes a set amount of time to burn. Depending on the RPM, the conditions for combustion, and other factors the ignition needs to be varied to be sure that the point at which the expansion of gases happens just after the piston starts to go down in the bore. Due to changing conditions in the engine, the timing should also be changed to suit all conditions.

How should the timing be varied?

Modern two stroke motors are funny things because they only start working well in higher RPM ranges hence the sudden power band. A very basic guide is that at low RPMs the exhaust gases are not well scavenged out of the barrel before the new charge is sucked in. This means that there is combustion air is "dirty" and takes longer to fully ignite the charge as the incoming air/gasoline charge is mixed with exhaust gases. As the revs increase the expansion chamber starts to work properly and sucks out the exhaust gases allowing a much better combustion in the barrel and more power. It also takes less time for the charge to fully ignite. Because of this inherent effect in two strokes the ignition curve would need to be further away from TDC when the engine is at low RPM and closer to it at high RPM. The catch is that if you go to far with the timing the heat can buildup very rapidly and fry the engine, you can get detonation,

Our idea (or really, my brother's - I am just among for the ride) is to intercept the trigger pulse off the electronic pickup on a P200E stator. Using a small processor running of the 20V AC power to the CDI we plan to be able to dynamically advance or retard the standard ignition to create a curve depending on the RPM of the motor. We hooked up an electrical meter to a standard P200E engine to find the trigger pulse and find a way for the processor to recognize it.

Above is as creen shot from the Fluke. From the high peak to the high peak is a 360 degree rotation of the flywheel. The high peak is the trigger pulse and the other low peaks are the other magnets as they rotate past the trigger pick up. Now we have a clear trigger point we can start working on a way to measure engine RPM and a way to delay/advance the ignition pulse very slightly to change the timing. An engine at 6000 RPM has 100 trigger pulses per second so the flywheel turns 360 degrees in 1 /100th of a second...so to change the timing one degree we will only have to delay the trigger pulse by 1/3600th of a second - thank god for the processor!!

Thanks to my big brother this will probably work, but we have no idea what kind of rewards it will give. We plan to try it on a standard P200 motor with an exhaust gas temperature gauge and a thermocouple cylinder head temperature gauge. If all goes well and once the kinks are worked out we'll try it on a Vespa Malossi P210 and a TS1 225 Lambretta. We'll dyno the bikes and post the results here.

More to come soon...