Jet valves usually burn out from running too lean but it is possible it seized for some unknown reason. Is this the first time you've pulled the rocker cover off to inspect it? Maybe someone in its previous life over adjusted it when trying to set the clearances... How much did it fail emissions by? That one dead jet valve = 25% less exhaust gas recirculation into the engine (when looking at how much ex gas enters the intake system it isn't a lot to start off with, but there wouldn't be an allowance between a pass and fail)