How does the cdi Know when the engine is on the compression stroke and not the exhaust stroke when it fires the spark plug, it only fires every other revoltion right?. The reason I'm asking is bacause I have to reasemble my engine after some transmission work and when it comes to installing the cam's should I set them to compression or exhaust, both setting will be correct as long as the piston is at TPD but when will the ignition fire. There must be something that I don't understand. Please help!!