273-4 vacuum leak

Hello to all. I'm writing to ask advice about a (likely) vacuum leak. I have a '67 Barracuda Convertible with a recently-rebuilt 273-4 Hi-Po (not by me; a local machine shop), .030 over; Hughes did the heads (ported and "larger" SS valves). Compression is estimated at 9.2. Cam was size recommended by Hughes staff (can't think of specs at the moment; it's a 'mild lope 340' from Comp Cams). Exhaust is from '69 340 Engine. Carter 4-barrel is the original PN (new from Edelbrock in '02); rebuilt a few weeks ago. We've replaced both gaskets (one over/under the 1" aluminum spacer), checked to hoses (advance, gauge and PVC hose). Engine is not fully broken in yet (two 20-minute runs); this is just the "assembly" stage! We've now started setting it for street break-in runs. Engine RPM is extremely slow to return to set idle unless I hand-cover the carb (which then immediately returns to set idle). Any advice? Thanks.