I have a 253fmm engine. I am using a cheap generic GY6 orange CDI, AC version. I have about five flywheels from different engines and all of them are slightly different and I don't understand why.

Some of the flywheels might be from things like Jinlun or Regal Raptor 125 twin so that would be 244FMI, others might be from some CG clone, but they all seem to have different knobs on them (is it called a boss?) that I assume cause the CDI to fire when it passes the sensor. But they are all different so why would that be?

Surely if the cdi is a simple AC box and only fires when the knob thing passes the sensor then there should be an optimum point for igniting the CDI regardless of whether it's a single or a twin engine, a 360 crank or a 180. I would think all these engines need to fire the spark plug at the same optimum point so why are all these flywheels different? I can interchange the CDI boxes and they all seem to work the same but different flywheels make the bike run differently.

I don't know which ones to use because I've built the engine from many bits and I never had a standard flywheel and CDI to start from.

Surely I can use for instance a 244FMI flywheel on a 253FMM engine because those engines are virtually identical apart from con rod length and piston size. And I would think that a CG clone flywheel should be no different than a twin engine flywheel, so why are they different? Do the CDI boxes fire at different times or something?

It's all very puzzling to me and rather time consuming to keep changing flywheels and CDI's to get a "right"! combination.

What do those adjustable CDI boxes do? Can I adjust the timing with one of them to compensate for using the "wrong" flywheel?