The valves cannot hit the cylinder head in that engine due to incorrect timing, Chris, I do not think that is the problem. (It is sometimes very different though, on some OHV or OHC engines, when the valves can hit the piston with devastating results. That is why it is a good idea to replace the rubber cam drive belt in certain car engines at the recommended intervals.) However if the camshaft timing is out by one tooth, the engine would run badly if at all.
