I think that maybe the rockers were over adjusted and the valves were not closing. It happens. It just does not make sense that an engine will not idle because the valves are 'tight'. As long as they are seating and sealing, and then opening to let air/fuel in it should run and if they open slightly more, particularly at idle, it would only be a matter of closing the carb barrel slightly more to lower the idle speed.

I have seen engines with the rockers over adjusted so tight that the valve will not seat and the engine will not run, period. But I have never seen an engine with the valves seating properly refuse to run because the rockers are adjusted tight. In contrast, a loose rocker setting will retard the timing of the engine, and make it harder to start and also it will develop less than rated power because of the retarded timing and reduced amount of fuel/air being consumed.