what is a cdi unit on a cg125

[shager]

what is it and what does it do?

[stinkwheel]

It is a Capacitor Discharge Ignition, usually taking the form of a 'black box'. There is a sensor on the end of the crank, every time the engine rotates it sends a 'pulse' to the CDI box. This makes a bank of capacitors (power storage devices) in the CDI box which have been charged up by the alternator send a bigger 'pulse' of electricity to the ignition coil which in turn makes the spark.*

So, the CDI delivers the power to make the spark in response to a sensor on the engine thus ensuring it sparks at the correct time.

Older bikes had points ignition instead. This is a kind of switch that is mechanically opened and closed by the rotation of the engine.

A CDI is in theory more reliable and the timing of the spark is fixed whereas points need 'tweaking' every so often to keep things working.

*Not strictly true, but it is easier to think of it like this.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.