The Stovebolt Site Forums The Shop Area Engine Shop low manifold pressure ?

I know you think the leak and idle circuit concerns can be ruled out but I think that is where the trouble will be found if the engine seems to run well off idle as you say.
Vacuum leak probably.
Consider the valve guides also when searching for source of leak.Probably not enough wear for a leak of this volume but I once saw a guide come loose and break up causing a massive leak.