On the slipped timeing - not to differ, but I would say that's not the issue. If it had slipped enough to not let it run, when it was I would think it would be running poorly, even popping through the carb or backfireing, let alone haveing a valve wack a piston by opening when it shouldn't.Maybe a fuel line isn't tight and it is sucking air on the pump inlet side , or your needle valve at fuel inlet to the carb is stuck , sounds fuel related , as in it runs out of fuel....... ? Just my thoughts.