Will a weak cdi unit cause an engine to idle rough?

Yes, a weak CDI (Capacitor Discharge Ignition) unit can cause an engine to idle roughly. The CDI is responsible for delivering the spark to the engine's ignition system, and if it is malfunctioning or providing inconsistent spark, it can lead to poor combustion. This results in uneven engine performance, including rough idling. Other factors may also contribute to rough idling, but a faulty CDI is a common culprit.