F.I. B2200 running poor

Ok...new crate engine, fuel injection, first test drive and the truck went from idling and reving fine to hardly or no idle, and no power during tst drive, runs like a wore out lawn mower...mostly when you give it 1/2 pedal or more. Checked plugs, wires, timing...all seem good. Fuel pressure regulator...when we pull the vacume line that runs from it to a small plastic peice on the fender well and plug that vacume line it smoothed out and helped a bit....was able to get the truck up to 65 mph with some still poor running but at least a bit better. Anyway....any ideas? We are suspecting the fuel press reguator but those things hardly ever fail. thanks all

vacuume leak???..perhaps