Thie following is how I understand the ballast resistor ignition to work. I'm quite happy to be completely wrong. In a ballast resistor system, the coil is one intended to have under normal running a reduced voltage (something less than 12V, typically 7 or 8V or so) across the low voltage side. That's achieved by putting the ballast resistor in series. While the starter is in operation a relay is closed, shorting the resistor, so the 12 volts (or whatever the battery can manage to deliver under load from the starter motor) is applied to the coil. This obviously helps the coil to breakdown the spark gap and deliver a decent spark during cranking. The coil is quite reasonably specified in terms of its resistance, but what's important is the voltage the low voltage side can endure under normal running without overheating.

Jeff, Sorry I wasn't clear. The second paragraph about opening and closing the throttle wasn't intended for you, it was just a general comment about starting engines with SUs, mainly old Minis with little compression and a friend's Morris Minor that sometimes seemed to have a mind of its own.

 

Re ballast system starting
The point being that even a good battery will pull down to 8 to 10 volts while cranking a starter motor due to internal impedance so a coil designed for 12 volts will be producing reduced output spark voltage.
A coil designed for 8 volts will be in its correct operating range and give a healthy spark.

Please note that regional and country differences may make my use of a nominal 8-volt coil seem strange. However, that is the way it was taught to me.

Cheers

 

Reporting back. It's been a few days - COVID and other life struck and I wasn't able to go through much diagnosis after getting the battery charged back up. I just went out and tried again after really doing nothing since the attempts over the weekend. Note that usually it starts on first attempt. It's not unusually cold here right now (55F), so while I do admit I need to work on resetting the mixture for the season still, I don't think that is at issue today.

So, first attempt with choke manually set on with the in-cabin switch added by a PO, it partially fired but still didn't want to start. Next I tried with wide open throttle as Glyn suggested and after one false try, it fired right up. I let it get up to temp and turned the choke off and it continued to run fine. After shutting down, I checked out the air tubes in the auxiliary carb and now the fuel is not visible. From this, I'm concluding that I was just flooded, which is maddening that I didn't think of that last weekend. I have the blue covered SU book in hand now and am reading up. While I've had to use WOT to start other carbureted cars in the past, I've not ever had that experience with an SU setup but then again, this is my first experience with the auxiliary carb.

Thank you all again for the help. Hoping this post helps someone in future.

 

Adding a marked up picture of the air tube I’m referring to in the auxiliary carb. It’s hard to see but you can just tell from the reflection that the fuel level is about 1mm from the top.

 

Delighted she started with WOT ~ Wide open throttle. Signs of too rich or flooding.

The Thermo range with AED are tricky on float bowl level setting on the feeder carb & Needle adjustment for ambient conditions. 42/45 in first diagram (setting how lean or rich the AED is running in ambient conditions at cold start.). The reversible arm on the set up is fairly minor on Thermo carbs used by Jaguar.