there should be vucuum at idle beneath the carb next to the adjustment screw there is a port.I'm sure this is where I will pick it up at. I will let you know what I find out at the junkyard. The engine is a big pump and when it draws in air thru the intake that is your vacuum if the throttle is almost closed (at idle)the vacuum is at its highest as if you had your hand over the opening. at wide open throttle the vacuum is lowest as though you moved your hand away and the pump can suck air freely. that little port is into the throat of of the carb and if you dont get manifold vacuum there the port is probably plugged