I have a 2003 400ex with a fresh top end new valves, cam, timing chain, and piston. I have had no issues with it since the rebuild approximately 15 engine hours ago. I replaced the valve cover gasket with a metal one since the paper one I installed during the rebuild was leaking, and I readjusted the valves after as a precaution. But when I went on a test ride to check for leaks it ran strong for about 2 minutes then shut off on me randomly under normal riding conditions nothing strenuous. When I pulled it apart it had bent both exhaust valves the left worse than right. What could of caused it I adjusted it at what I thought was tdc on the compression stroke and it ran fine when I started it up to warm up no weird noises whatsoever. Please help I can’t think of anything else that could of caused it.