are you rotating the crank to the left the whole time to get top dead center?? if you turn the crank back to the right the tention will be let off of the timing belt resulting in an ark between the cams if you have a DONC motor and loose on the accesory side of the belt of a SOHC motor. As long as the tentioner pulley has the spring connected as it should be, the belt will be tentioned if the instructions are followed correctly. I think you are supposed to rotate the timing belt like 4 or five times with the tensioner loose and then tighten the tentioner down. This is what I recall anyhow.