I don't think the intake manifold is your problem. A wideband would go a long way towards helping this.
Sounds like you either need more pump shot, a different style of squirter, or perhaps the idle mix is too lean. Have you messed with the squirters at all?
I am guessing the HP doesn't have a timed vacuum port? I'd remove and cap that vacuum line until you get the problem figured out.
HP 650 is a lot of carb for that motor. It could maybe might be air bleeds. Maybe.
