Thank you for your PM but question and answers should be placed in open forum - otherwise the real purpose of the forum (open forum information sharing), would be defeated. Thank you for your kind words though but you should read my signature - in addition I have not had a 400 for over 10 years so I can no longer profess to be at all familiar with them.
I take it your question concerns the 400 unit you are incorporating into the 1957 BMW Isetta?
Where are you located by the way, your profile is not complete? We encourage new members to do this and introduce themselves in the 'who are you, where are you from?' section.
I have read all of your posts on your project and you are clearly very technically competent and I hesitate to offer my meagre advice.
But first a couple of scene setting questions:
1. Have you ever witnessed this specific engine, that you are now fault finding, running?
2. Have you cranked the engine with the spark plug removed from the cylinder but connected by its HT lead and with the plug grounded to observe if it indeed producing a spark?
3. Have you ever done a compression test?
4. Have you read this thread and the successful conclusion of it?LINKY
I am clutching at straws but aware you have done exhaustive (and no doubt frustrating) work already.