maybe we are barking up the wrong tree or maybe yours could just be a case of the swirl valve sticking until a certain vacuum is reached, have you tried taking off the vaccum pipe to the swirl valves and trying it if that works i would take off the manifold and operate swirl valkves on the bench to observe there operation, worth a go i think as this will only actually cost you time and not any parts. I scoped my amm and could not really see anything apart from the digital waveform but just looked on pico site and seen the massey method so i will possibly try that next time has anyone else tried the massey method
regards
carl