The jet valves only act as a secondary inlet to the combustion chamber from the inlet manifold so it "shouldn't" cause blow by - unless something scary has happened like a crack between a jet valve gallery and an oil gallery has occurred (don't know if this is even possible...) 4G54 MCA-JET valve heads weren't sold here in Australia - and the few mechanics that have run into them do things like weld the combustion ends of the jet valves shut and cut the auxillary rocker arms off due to their lack of understanding of how they work. Their excuse usually goes something like "they burn out anyway..."