When your doors are completely closed, how close is the operator arm to the top of the shaft on the bullwheel? you should have about 1/8" of shaft showing above the arm. If the arm is below the shaft, the door will drift open. If you shortened the stroke, and increased the length of the arm, you will close in about 120 degrees of bullwheel movement. You should be using about 270 degrees of bullwheel motion.