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Look hard for a vacuum leak on that side of the engine. Fords are notorious for sucking holes through the rubber connecter at the manifold vacuum sources. Sometimes they are hidden real good underneath the intake. Listen for the hissing and spray some carb cleaner in suspected areas listening for rpm change when you hit a leak.
You should also test the O/2 to be sure it is capable of reading rich but you'll need a scanner or DVOM to do that.