This still sounds like a carb problem to me, although I did have some spectacular problems with the coil ignition on my own 1948 4G from time to time. As one last check on the Solex carb, have you undone the 4 screws which hold the float chamber section to the rest of the carb? This joint is vertical. (I don't have a bi-starter carb here at present so that description is from memory). When you remove these 4 screws, you should be able to see the passage which takes fuel into the bi-starter. There is a hole in the gasket corresponding to this passage. Could the gasket be fitted such as to block this passage? I do emphasise that I am working from memory here, but it is worth a quick check. Presumably the errant silicone could also have got into this passage?
If we have no joy with the carb, we can go through the coil ignition issues but I will need a good description of the coil conversion to be able to help.
Changing valve guides, etc., at this point is most unlikely to solve the problem, in my humble opinion.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.