Small Engines (Lawn Mowers, etc.): 18hp Briggs twin IC carburator wont adjust, air mixture, orfice

Question
I have an 18hp Briggs Model 422707 twin IC engine.  I recently rebuilt the carburetor and am having difficulty adjusting it.  When I turn the air mixture screw clockwise the drop in RPM's is noticeable however when I turn it counter clockwise I have no drop in RPM's.  What am I missing?  It runs half way and will power the deck but drops the RPM substantially.  I checked the float and it appears to be ok.  Any advise would be appreciated.

Thank you
Rick

Answer
Have you checked the engine speed with tachometer?  It should be 3300 RPM.

Does the engine run smooth at idle and high speed with the blades off?  You did not mess with the governor adjustment or spring while rebuilding the carb?  Did you clean the carburetor main jet and jet orfice with compressed air from an air compressor?

Let me know.
Eric