Last year I bought a JD GS-30 wilth a 36" deck. It was used and I did not get a manual with it. I find that it pulls to the right requiring constant adjustment to keep it going in a straight line. I can't figure out how to fix it. There must be some adjustment to make the left drive wheel go slower or the right wheel to go faster. It's probably a simple thing but I just don't see it. Can anyone help me with this?