The last post on this topic was last fall, but I expect that the mowing season will have folks looking at this subject once again. At least I have been back to trying to understand why it is necessary to put the "CAM knob" in the highest position. After laying on my back staring up into the underside of my tractor looking at what the knob really adjusts and the rockshaft that raises and lowers the mowing platform, the culprit is two loose bolts that hold the cam mechanism onto the frame of the tractor. This has been driving me crazy since I bought the tractor over a year and a half ago.
I've uploaded a file from the JDparts.com, you can find it at:
http://jdpc.deere.com/jdpc/servlet/com.deere.u90490.partscatalog.view.servlets.HomePageServlet
If you look at the drawing, you can see the hunk of metal that sticks up off the rockshaft. On the left of the hunk is a small protrusion that SHOULD come up against the cam when you set the heighth and lower the platform. On mine, the bolts that hold the cam onto the frame has loosened and that bar was hitting the cam above that protrusion. That allowed the platform to come down about an inch and half, rather than the .5 inch that is spec.
To fix the problem; take off the platform, raise the lift as high as possible, turn off the tractor, (there is a shaft that runs between the engine and transmission that doesn't mix well with fingers). Loosen the two bolts that hold the cam mechanism onto the frame, so the cam comes back to a normal position and then just tighten up the bolts. BUT, and this is big, there are no lock washers on the bolts -- doesn't seem to be room -- so make sure the bolts are really tight, but don't snap them.
Now I have lots of height adjustment.