So you put a bigger valve in it and a big cam with a lot of duration (late IVC) without the necessary compression which then messes with the vacuum generated in the port and then what what does the carb see at the throttle bore and the venturi? Its all interconnected but people never spend enough time thinking about how one change effects the system as a whole.
The "bigger" the invitation for reversion the further up the intake tract it will travel further interrupting the vacuum generated at the booster venturii will it not? What does that do to cylinder fill?