I tweeked my cam timing to a little over 6 degress retarded on the exhaust cam at the weekend, i turned the car over numerous times by hand, there was not contact between pistons and valves. At first i thought it was fine but i started it again last night and there seems to be a very minor miss fire, have i bent a valve or can the retarded timing cause this. It revs fine, its idle is just a little unstable. I have just fitted underdrive pulleys could it be these?