You have to add twists to the string to shorten it. Or you could take a few twists out of the cable. To do this you can use a press or back out the limb bolts till the string goes slack, but be sure and count the turns you back off the limbs.
The problem with having to small a gap is that when shot the bow doesn't reach brace and stop. The string over travels and in turn the cam rolls closer to the end of the groove. If it gets too short the string can run out of groove and slide off the cam. Sort of like a bicycle chain rolling off a sprocket only a lot quicker and more violent.
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