1 1/2 turns is the starting point, there isn't a spec measured in turns. With the engine idling warm, slowly turn one screw out until the rpms or vacuum are at their highest point, repeat on the opposite side. Recheck both screws by turning them in and out slightly to get the leanest, best idle. If you have the screws 3 1/2 turns out your jets are too small. 18 mpg is not impressive, I averaged 16mpg all summer with the 351 4 Just bolting a plate on in place of the EGR valve doesn't mean the spacer isn't corroded through. Remove the carb, remove the spacer from the manifold and inspect the underside. We used to replace a bunch of them for corrosion, it got to the point where Ford released cast iron replacements for the aluminum spacers on V8's.