No spark and a clunk

[leewiffen]

My 1977 911S will not start. Was running fine till it got on the motorway then cut out. Cooled down, started, then cut out again. Recovered to home, prior to being able to get it into a proprietary specialist, I thought I might try to educate myself
I do get a whine from the Cdi box, however I also get a loud clunk which is a worry. I've put a short video on youtube
Here's the link....https://www.youtube.com/shorts/PP1LmAlxCRk
any ideas?
TIA

[Gary71]

Sounds like the solenoid is firing but the starter isn’t turning.

Some quick ideas:
- Flat battery
- Bad earth. Either at the battery or the box to body strap under the car
- Starter has died!

[Gary71]

But based on it cutting out it maybe that the electrical issues are all related and the starter is just a side effect

[leewiffen]

Hmmm,
The engine spins when I do try to start it, so it's not battery or starter motor. But you could be right that the clunk is an after effect of some electrical issue.

[Jonny Hart]

That sound isn't coming from the CDI and it isn't the starter either. My bet is the car has the electromechanical voltage regulator which has most likely failed. That might have led to the voltage going too high for the CDI box and it destroying itself. The earlier voltage regulator has contacts which could be the noise you are hearing when you turn the key. Get a helper and try to pinpoint the sound.

[MikeB]

If the CDI is working it will make a constant sound once the ignition is turned on. So turn it on and go to the back of the car and it should be whining. If no sound then follow Jonny’s advice

[Jonny Hart]

If the CDI is working it will make a constant sound once the ignition is turned on. So turn it on and go to the back of the car and it should be whining. If no sound then follow Jonny’s advice

True, but could be misinterpreted, hence:

1. Working CDI makes a whining noise when the ignition on.

2. Whining CDI does not imply working CDI. The whining is the CDI's power supply but other parts of the CDI circuit can fail.