Go the trinity 2-1 35mm setup, had the same problem - one of my reeds had partially disintegrated, and that cylinder was the one that was not firing at idle.
If you have ruled out spark, as not being the cause, and it was all working perfectly with two carbs, and one cylinder is firing with the single carb set-up, you are pretty much left with only between the carb and that cylinder as being the likely problem.
Check you reeds and intake boot.