Hmm.. are you sure?
I have heard this but I'm 99% sure Stu @ MSD wrote a article/post about MFI ECU's and they do not control the boost like that, the Amal is always open as long as it has 12v and an earth and makes a very loud click when it opens/closes you'd here it clicking like mad if it was PWM controlled and I have never heard mine (driving or on the RR) when I had one fitted.
AFAIK they are not like our ICV's which have varying degree's of open/close controlled by PWM, Amal's are either on or off

Ref the ports... I have never reallly looked at one closely as I replaced mine with a AVC-R and never tampered with it prior to fitting that.