So, according to the wiring schematic, ignition coil 1 (front?)'s primary coil's ground is tied to the tachometer.
That means that when working properly, the CDI box is providing a temporary path to ground each time a spark should trigger.
If the tach is dead, there's no path to ground, which means either a broken wire between the tach and the CDI box, the CDI box has failed, or the pickup coil that triggers the front cylinder has failed or has an open wire (black/yellow, black wire pair, lower pickup when looking under the engine cover).
I guess it's also possible that the primary coil in the ignition coil might have failed 'closed', somehow overpowering the pulsed signal, but that's unlikely.
Last edited by Thorn; 07-25-2019 at 02:53 PM.