had no spark on piaggio liberty 50 so decided to post this guide to what you should read from each wire with a multimeter. 1. Disconnect the CDi Unit from the connector block and test from the block. 2. Check between the white (earth) wire at the cdi connecting block to a good earth, should read O Ohms the multimeter will rise on contact then quickly settle at 0 Ohms, or very close to 0 ohms depending on the sensitivity of the meter. 3. Check between the red (pickup) wire and good earth. should read between 85 - 140 ohms. 4. Check between the green (exciter) wire and good earth. Change multimeter to AC voltage, Switch ignition on, crank the engine, should read about 80V AC no less than 50V AC. Switch ignition off, change back to Ohms, should read 80 - 1000 Ohms. the diagram I posted is not from a piaggio as the wires are entering the CDI box in a different order but is the simplest I can find to explain the principle of the CDI.