Rich running, waveform analysis

1973 E-type V12
Hi all, never thought I'd be using a Pico on such a car but really could use some advice with the ignition waveforms. It's running super rich and was suspecting ignition system, so scoped primary vs secondary coil voltage as well as primary current and voltage. There are two things that are puzzling me which I'm sure are linked. As the pickup in the distributor grounds the coil, we can see the voltage drop to 0V, great. Then around 3ms later it raises to around 8V, which I can't understand why?
https://ibb.co/NKSw4FL

Looking at the current, it plateaus at the moment the ground goes from 0 to 8V.
https://ibb.co/1RzHHRT


Pickup sensor:
https://ibb.co/8cCpZ20

I can send the amp away for testing but I'd really like to know what the issue is first.
Massively grateful for any help.

muz122 wrote:
Looking at the current, it plateaus at the moment the ground goes from 0 to 8V.
https://ibb.co/1RzHHRT

The driver providing the ground is intentionally providing a bad ground to limit the current flow. You only need so much current to saturate the coils, any more current beyond that is not needed and just creates heat. That plateau and the ground jumping to 8v is directly tied. My guess is the ignition control module is doing it on purpose because, well, the coil has had it fill of energy, and it's holding it until it's ready to let go of the ground completely. When it lets go of the ground the amps drops to zero and spark occurs.

Have you looked at your spark plugs?

The way the Spark line starts at around 5 Kv, then drastically drops to about 2.5 KV makes me think the plugs are carbon fouled.

It takes more Kv to break through the carbon, but less Kv to maintain spark once the carbon has been penetrated.

I assume it is on standard 2 Zenith-Stromberg carburetors per bank of cylinders . These were really simple carbs but need regular maintenance. First check the pistons in the carbs are rising an falling freely, if you lift the piston fully up it should slide down smoothly and freely, if the piston damper removed it should slide down stop with a distinct "click". If that is okay before going any further I would obtain 4 new diaphragms for the carbs as once the top half of the carb is removed the used diaphragms won't go back into place.

Well spotted, they are carbon fouled but cleaning them is a temporary solution as they soon foul again due to the rich condition. I'll be renewing them very soon (would like to get it running a bit better first).
Many thanks Chad

Correct, I've overhauled them and tuned as best I can but still getting backfiring and rough running. Pulled 2 of them off again last night to see if I'd missed something stupid, but everything is in place and all settings (float height etc.) are within specs.
The thing that got me onto investigating the ignition is when setting the timing, I was getting an intermittent strobe from the timing light on cylinder 1A. The spark on all cylinders isn't what I'd call strong and blue but I don't have another E type to compare to:
drive.google.com/open?id=1xeSHqgeLOhKphitrN_F9SNtP3SlAGbQo

Many thanks