The 400 had two petcock types but I don't know about the 350, I've only worked on a couple and don't recall anything about the petcocks. The earlier 400 used a petcock with a larger "bung" thread, a fairly complicated unit that was obviously designed for two outlet hoses with one left unfinished and unused. Later models have a smaller bung fitting, are physically smaller and definitely designed only for a single outlet. The later ones have the cover plate (holding the selector lever in) riveted on, early ones have screws - you can still remove the rivets and drill/tap for screws to service the internals. The internal valving is via a hard to describe tapered unit in the older ones. The later ones have a rubber plate with holes, turning the lever lines up a passage to let fuel flow.
Neither will empty the tank, because there is no crossover tube linking the side lobes of the tank. You can get a few more miles after running out by tipping the bike way to the left, letting the fuel in the right bottom lobe of the tank flow across to the left where the petcock is. Many earlier Honda tanks had a crossover tube that links these lobes so "empty" really meant empty but they also had deeper lobes. It's a pain though when you wish to remove the tank, it has to be emptied to remove the crossover. Plus the extra "desperation reserve" can be useful.
