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Looking for someone to offer advice on vintage Solex carb

I've spent the past 2 years restoring a VW-powered aircraft and I'm struggling with getting the engine fired up for the first time.

Part of the challenge is that I'm re-using the carbs previously fitted, which are 'vintage' to say the least: Solex HF "Assembly 22", seemingly a popular choice for Citroen Traction Avant vehicles pre-1956:

https://traction-owners.co.uk/wp-content/uploads/2017/01/Solex-VB-HF-AHD-booklet.pdf

New Leburg igntion fitted, all gaps set correctly, nice big sparks, brand new fuel pump, carb bowl containing fuel. All is good.

There appear to be quite a few quirks to starting from cold with these carbs and so posting here on the extreme off-chance there may be an older / wiser pilot on here who has some experience of these carbs and can offer advice / wisdom on how to set them up correctly to help things along.

These were the original carbs fitted on the a/c (which last flew 12 years ago) - they have something called a "bi-starter" (effectively a carb within a carb) that means things need to be set-up spot-on to start correctly from cold.

I could replace the carb, but these were working fine before and I want to perserve if I can...

This bi-starter mechanism is quite similar to that used on the Weber carbs (common for vintage Aston Martins), if this helps...!