Don't be afraid to bump up the idle timing 4 or 5 degrees more. That low compression engine with a very bad combustion chamber (open chamber with the spark plug far off on one edge of it) will be much happier with 18 or even 20 degrees idle advance. Your total initial plus mechanical advance should be around 40 degrees and another 10 degrees or so ported vacuum advance. Sounds like your advance is too retarded and you should fine tune it before swapping carbs because whatever carb you use is going to need more advanced timing....