I measured the voltage going into the CDI on pins 15 and 31/1. The traces are below.
The first trace is starting the engine; Key ON - 12V appears; Start - Voltage droops to 6-9V; Engine runs - Voltage goes to 14V with 2V spikes where the CDI fires.
Second trace is a close up of the 12V supply. The negative spikes are where the CDI is firing.
In the third trace the engine is starting to stumble then stall. The spikes disappears as the spark is not applied.
Next step is to monitor the inductive signal from the distributor. If the signal is consistent then definitely the CDI needs replaced.
