In an attempt to do my own valve adjustment, I hit a road block. I got everything off and went to turn the craftshaft clockwise to make my measurements by aligning the "T" index mark with the notch. My crankshaft won't turn more than 90 degrees in either direction, clockwise or counter-clockwise. In the range that I could turn the crankshaft, it made couple of loud pop and then started to turn until it came to a dead stop. I've removed the spark plugs, the camchain tensioner, and even put my weight onto it but nothing. The bolt would turn but the crankshaft stayed at the same position. Any idea what's wrong?