No spark when cranking, but tries to fire briefly when I release the key?

[ischmitz]

In the video the first and the second time the OP cranks you can see the engine (fan come to a complete stand-still) when the key is released. Nothing is moving and then almost a second later each time a single spark appears. This is consistent with a damaged CDI box.

Ingo

[Jonny H]

^. I'd agree. The box shouldn't ever spark without an input signal. The 6 pin expects a zero crossing signal from the VR in the dizzy. There's a slim chance it could be the VR coil but I doubt it.

[yelcab1]

I lean in that direction too. My last CDI on a 71T was buzzing like it is good but it was bad. A swap to a rebuilt box fired right up. If you are sure all wires are correct, then the CDI is the bad part.

[gsxrken]

Happy to post an update for those following along and future searchers:
I had reversed two wires on the rear of the tachometer and incredibly that was enough to lock up my CDI box.

I'm sure I would never have figured this out on my own, so I owe a HUGE thank you to Bob Ashlock and his spot-on advice quoted below:

"Consider the following: The TD pin on the CDI is the tach signal. This pin is intimately tied in with the internal trigger circuit in the CDI. I believe your car also has a Speed Relay, and the TD signal ALSO goes there. If something is honky with the TD circuit, it can freeze the trigger pulse circuitry in the CDI as I will explain below:

When testing a unit on the bench that is running normal, , if I apply (+12V) to the TD pin, the unit will STOP. If then shut off the +12V feed to pin 15 (as occurs when you let off the key), but keep +12V applied to the TD pin, and wait a bit, the moment I remove +12 from the TD pin, the unit will spark ONE time. That is because the discharge cap is charged and the transition of the TD pin triggers the SCR one last time to dump energy into the coil.

SO, the first experiment I would do is REMOVE any signal from the CDI TD pin and allow it to float. Cut the wire if you have to. Then see if the engine will start up. If it does, then the Tach itself, or Speed Relay, or something phantom must be forcing TD to +12V. The attached diagram only shows the TD signal going to 3 places: CDI, Tach, Speed Relay. spark ONE time. That is because the discharge cap is charged and the transition of the TD pin triggers the SCR one last time to dump energy into the coil.

SO, the first experiment I would do is REMOVE any signal from the CDI TD pin and allow it to float. Cut the wire if you have to. Then see if the engine will start up. If it does, then the Tach itself, or Speed Relay, or something phantom must be forcing TD to +12V. The attached diagram only shows the TD signal going to 3 places: CDI, Tach, Speed Relay."

Based on Bob's advice, I pushed the TD pin out my CDI harness plug and cranked the car, and boom, plenty of spark during cranking! This led me to suspect the tach, review my notes and photos and notice that I had two tach wires reversed... Switched them over, reconnected the TD pin in harness, and car now speaks during cranking.

One for the collective knowledge Pelican database.