With the Grand Quilter on the frame, the needle is threaded from front to back, as viewed from the front end of the machine. So when you're pushing the machine toward the back, the thread is actually coming out from "behind" the needle and feeding around the needle toward the front as the machine moves away from you. (You need to visualize this.) That means there is far more tension on the thread in that direction, as it has to feed through the eye toward the back and then pull around to the front as the machine moves toward the back. This creates a lot more stress on the thread in that direction than the other directions.
That's why you will want, not necessarily a larger needle, but a needle with a larger eye and a better guide groove to let the thread flow better. Thus the recommendation for a topstitch needle, and one that is "slipperier" thus the recommendation for a titanium coated needle. Going to an extra large needle for the thread size, like a 16 for a 50 wt thread may or may not give you a bigger eye but it may introduce other issues.
Like the other poster said though, always just make one change at a time and test between each one.
Last edited by QKO; 12-12-2013 at 11:48 AM.