Here are a couple of links to others doing a conversion to Weber's on a 2.7.
What a difference...
Weber jetting for 2.7
Weber question
I have a 2.2S and have the following setup:
30mm venturis
60 idle jets
125 mains
180 air correctors
f1 emulsion tubes
I want to go to 32mm venturis and see what that does.
From your symptoms I would thing the mains are too large, but you need to find out what the air correctors are. I think the answer lies in the main, air corrector and emuslion combo.
Another option is to send your specs to PMO and see what they recommend.