Someone please help. I have a 1420 with grain rotor, smooth seperator bars, and keystock grates. I am having heck cutting beans this year and can't figure out why. When sun goes behind clouds for a few minutes the combine can tell. I am constantly getting that rumble in the back and have to go extremely slow (1-1.5mph) . When beans are dry I can run 3 to 4 mph. My vanes are set in mid position and are not movable because of aftermarket rotor cage. Why does this small amount of dampness slow me down so much and what can I do to help this?