Sorry BB can't help much, I just can't afford the time to mess with these machines, spend a million hours and still end up with the same end result. 90% of the time the carby is the fault and just because you have 2 carbys doing the same thing probably means the same thing is wrong with both carbys. Have you tried holding your thumb across the carby throat in various positions before you try opening the throttle, Cutting out as you start to open the throttle indicates an air/fuel mixture problem and varying your thumb position can prove that
